A Conchological Manual by George Brettingham Sowerby (early reader books .txt) 📖
- Author: George Brettingham Sowerby
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spire only taking place after the few first volutions.
SPIRAL NON-SYMMETRICAL UNIVALVES.
As these constitute the largest class, it will be necessary to dwell upon
them in detail. First as to _measurement_.
[Illustration]
The length is measured from the apex, to that part of the aperture _a_
(fig. 24), at the greatest distance from it. The _breadth_ is in the
opposite direction. The _anterior_, or front part of the aperture, is
marked _a_, where the head of the animal protrudes.
_Spire of non-symmetrical Univalves._
[Illustration: Fig. 25, _obtuse_; 26, _acute_; 27, 28, _decollated_; 29,
_concave_; 30, _papillary_; 31, _mammellated_; 32, _discoidal_.]
In counting the whorls of which the spire consists, we commence at the
apex, and reckon downwards to the last, or body whorl. The spire is
described as being long or short in relation to the aperture: in which
case, all that is above the aperture is measured with the spire. Its apex
requires particular notice, as the character of the whole shell frequently
depends upon the particulars observable in this part. It is sometimes
_obtuse_, or blunt; sometimes _acute_, or sharp. In the Cones it is
frequently flat, and in Planorbis it is concave. It is sometimes of a
different structure from the rest of the shell, retaining the horny and
transparent appearance which characterized it when the animal was first
hatched. The Tritons present an instance of this, although it is not always
observable, owing to the tenderness of the substances which causes it to
break or fall away in many specimens. A very remarkable instance also
occurs in Bulinus decollatus (cut, fig. 27, 28), so named, because the
apex, to the depth of several whorls, falls off, and the shell is
_decollated_. In this, and many more instances, among Pupæform land shells,
the occurrence of this circumstance seems to be by no means rare or
accidental, a provision having been made for filling up the opening by a
septum. A _papillary apex_ is one which is swelled at the extremity into a
little rounded nob, or nipple; and a _mammellated_ apex is one which is
rounded out more fully into the shape of a teat.
_Whorls._
The spire is described as consisting of _numerous_ or _few_ whorls, and
sometimes the number of them is particularly stated. A whorl consists of
one turn of the spiral cone. The whorls are described as _flattened_, when
the sides are not bulged out so as to cause the outline of the spire to
deviate considerably from straightness: when the contrary is the case, the
whorls are said to be _ventricose_, and either _rounded_ or _angulated_.
The degree of rapidity with which the whorls become enlarged presents an
important source of distinction. The _suture_, or seam, which separates one
whorl from another is also noticed as being _distinct_ or otherwise;
_canaliculated_, or grooved; or covered by an enamel, which in some
instances is swelled into a ridge or _tumid_.
[Illustration: Fig. 33, _few_; 34, _numerous_; 35, _rounded_, _ventricose_;
36, _angular_, _ventricose_; 37, _flattened_.]
_Suture._
[Illustration: Fig. 38, _canaliculated_; 39, _enamelled_.]
_Varices._
Varices are caused by periodical rests or stoppages in the growth of the
shell, when the edge of the aperture thickens, and renders the shell as
complete as when full grown. Again, after an interval, another check takes
place, and another thickened edge is formed, and so on in succession, until
the animal arrives at maturity, and the shell is full-grown. The thickened
edges successively forming the aperture, remain visible on the outside,
through all the subsequent stages. When these rests take place at frequent
periods, the varices will of course be numerous as in Harpa and Scalaria.
They occur at regular or irregular distances, varying in shape and other
characters. When the varices occur at regular intervals, and form a
connected ridge from whorl to whorl up the spire, they are said to be
_continuous_, as in Ranella; when on the contrary, the varix on one whorl
does not come in contact with that on the other, they are described as
_discontinuous_. In order to distinguish a regular varix from a mere
external ridge, it will be sufficient to notice whether its edge overlaps
the external surface, and whether it resembles the open edge of the
aperture, which true varices do.
[Illustration: Fig. 40, _numerous_; 41, _few, continuous_; 42, _few,
discontinuous_.]
_Aperture._
The aperture or opening of the spiral tube, was formerly described as the
mouth; a term calculated to convey an erroneous impression, when applied to
a part of the shell which has no correspondence with the mouth of the
animal. The word _aperture_ is used by modern writers in a general sense,
including the cavity, its edge, and the canals. The cavity itself is
distinguished in various shells as to its shape, which depends much upon
the degree of modification produced by the last whorl. In some cases, as in
Cyclostoma, where the aperture stands apart from the last whorl, the shape
is round, or nearly so. The Scalaria presents a good example of this. In
others, where the inner edge or lip, wrapping over the body whorl is nearly
straight, the aperture is _semi-lunar_, or half-moon shaped: this is
remarkable in the "_Neritacea_" of Lamarck, named, on that account,
"_hemi-cyclostomata_" by De Blainville. In a great number of instances, the
lower part of the body whorl enters obliquely into the upper part of the
aperture, the result being a _pyriform_, or pear-shaped opening. The
aperture is described as _long_ when it is largest in the direction of the
axis, and _wide_, in the contrary case. The _anterior_ is the part at the
greatest distance from the apex, and the body whorl; the _posterior_, the
part nearest to the apex. Thus some apertures are described as _posteriorly
contracted_ and _anteriorly widened_, or the reverse. A _linear_ aperture
is one contracted in its whole length, as in Cypræa. When the whorls are
angulated, a _trigonal_ aperture is the result, as in many species of
Trochus. Some are _transversely oval_, that is in an opposite direction to
the axis, and others _longitudinally oval_. When the whorls are formed with
two outer angles, a somewhat quadrated aperture is formed. There are other
variations too numerous to mention.
_Apertures._
[Illustration: Fig. 43, Helicina, _semilunar_; 44, Pirena, _pyriform_; 45,
Cypræa, _linear_; 46, Trochus, _trigonal_; 47, Cyclostoma, _rounded_; 48,
Chilina, _posteriorly contracted_; 49, Stomatia, _transversely oval_; 50,
Murex, _longitudinally oval_.]
The entire edge of the aperture described generally, is named the
Peritrême, but this term can only be conveniently applied in cases where,
in some at least of its characters, it is the same all round, so that one
descriptive term is applicable to the whole. As, however, this is of rare
occurrence, it is found convenient in descriptions to separate the rim from
the outer lip. In a great number of instances, this is done naturally, by a
canal, or notch at the anterior or lower extremity, and by the posterior
union of that part which overlays the body whorl with the other portion. At
these two points the outer and inner lips separate from each other: we
therefore describe the
_Canals of the Aperture._
When there is neither notch nor canal, anteriorly or posteriorly,
interrupting the edge of the aperture, it is described as entire. When
there is a notch or sinus at the anterior extremity, it is said to be
_emarginated_. When the edge of this notch is expanded, and drawn out in
the form of a beak, it is said to be _canaliferous_, or to have a _canal_.
When, in addition to this, the lips are thickened and contracted
posteriorly near their junction, and drawn out so as to form a groove, it
is said to be _bi-canaliculated_, or to have two canals. The _anterior
canal_ is said to be long or short, according to the proportion which it
may bear to the rest of the shell. Thus the canal of Ranella ranina (fig.
393 in the plates), may be described as _short_; while that of Murex
haustellum, (fig. 396, pl.) is _long_. When it is wide near the aperture,
and becomes gradually contracted towards its termination, it is said to be
_tapering_, as in Pyrula (fig. 388, pl.); when the termination is sudden,
it is described as _truncated_. If, on placing the shell upon a plane, with
the aperture downwards, the canal is seen to rise upwards, it is
_recurved_. In Buccinum and Nassa it is turned suddenly over the back, and
forms a short, curved elevation; it is then described as _recurved_ and
_varicose_. If the edges meet, so as to form a tube, it is said to be
closed, as in some species of Murex and Typhis. The posterior canal is, in
some cases, _free_, or standing out from the spire, as in some species of
Ranellæ; while in others it is _decumbent_, running up the sides of the
spire, as in Rostellaria (fig. 402, pl.).
_Canals._
[Illustration: Fig. 51, Fasciolaria, _truncated_; 52, Nassa, _recurved_,
_varicose_; 53, Cerithium, _recurved_; 54, Typhis, _closed_.]
_Lips, or edges of the Aperture._
The part of the edge of the aperture next to the body whorl is named the
_inner_, or _columellar_ lip. Posteriorly it commences at the point of
union with the outer lip, where that touches the body whorl, the junction
being generally marked by an angle, and sometimes by a canal. Anteriorly it
terminates where there is generally seen a notch or canal, or sudden angle,
from which the outer lip proceeds. The part which setting out from the body
whorl, and proceeds outwards at a distance from the axis, till it reaches
the anterior canal or notch (or its place in case of absence) is named the
_outer lip_. In many cases the edges are united in such a manner, that it
is difficult to distinguish where the inner lip terminates, and the outer
lip commences: when this is the case, it is usual to describe the margin or
peritrême, as a whole, without distinguishing the parts. The _outer_ lip,
sometimes called the right lip, or _labrum_ of continental writers, is
sometimes acute, not being of thicker substance than the remainder of the
shell. In other cases it is _obtuse_, or thickened and rounded at the edge.
When thickened and turned backwards it is described as _reflected_; when,
on the other hand, it is turned inwards towards the axis, as in the
Cyprædæ, it is _inflected_, or involute. When it is _toothed_, a
distinction must be observed as to whether the dentations are external or
internal. If the teeth are small and numerous, it is _denticulated_; if
larger, it is _dentated_; when expanded into a kind of wing, as in some
species of Strombus and Rostellaria, it is described as _alated_; and a
family in Lamarck's system is named "Alatæ," from this very circumstance.
In some of those which are expanded, the expansion is divided into
separate, attenuated portions, they are then said to be digitated.
_Outer Lips._
[Illustration: Fig. 55, Helix, _reflected_; 56, Cypræa, _involute_,
_denticulated_; 57, Sera, _alated_; 58, Murex, _digitated_; 59,
Rostellaria, _dentated_.]
The _inner_ lip, sometimes named the _columellar_ lip, or "_labium_," is
subject to similar variations as to thickness, dentition, &c. That portion
of it which lies upon the body-whorl is frequently distinguished from that
which intervenes between it and the notch or canal. De Blainville,
restricting the term _lip_ "bord gauche" to the former portion, applies the
term "columella" to the latter; and in some instances this may be the more
convenient method of describing the part in question. The columellar lip is
sometimes _detached_ entirely from the body of the shell, as in Murex
haustellum; in others it is _decumbent_, or lying over the last whorl,
although quite distinct, and in some cases, _thickened_, _callous_, or
_tumid_.
At the lower or anterior part, sometimes called the _columella_, there are
in many instances flattened, laminated folds; these are particularly
conspicuous in the genera Cymba and Melo, where, being obliquely spiral and
laminar, they are extremely elegant, presenting to the eye graduated
repetitions of the line of beauty. In other cases, as in the Turbinellæ,
they are more horizontal and thickened.
In some cases the columella is swelled into a varicose mass; as in Oliva,
Ancillaria, &c.; it is then described as _tumid_ or varicose. It is
sometimes _tortuous_, and sometimes straight, and is
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