Part 5

In the above section of a Nautilus, fig. 13, diminished in size, showing the whorls and chambers (c), it will be seen that the edges of the septa (s) are formed in one simple curve. In fig. 14, the upper part of an Ammonite, the undulating line will be seen; and in fig. 15 a specimen is given of the arborescent septa.

Siphon.

The Siphon is dorsal when placed near the outside of the whorls; central when near the middle; and ventral when near the inside of the whorl, or that part which leans against the last volution. When it passes uninterruptedly from one chamber to another, it is described as continuous, as in the case of Spirula; when, on the other hand, it only passes through the septum a little distance, and opens into the chamber, as in Nautilus, it is discontinuous.

Whorls of Symmetrical Univalves.

They are disunited when they do not touch each other, as in the case of Spirula (fig. 471 in the plates); but in the contrary case they are said to be contiguous. In some species of Nautilus the whorls overwrap each other in such a manner that the early whorls are entirely covered by the last, the edges of which reach to the centre of the disk: the spire is then said to be hidden; as in the Nautilus Pompilius. In Nautilus umbilicatus the spire is nearly hidden, the whorls not quite covering each other; but in the greater number of the Ammonites, the largest part of the preceding whorls is seen. To express the degree in which the whorls overwrap each other, has caused much difficulty in concise descriptions. Perhaps it would be well to apply the term spiral disc to so much of the shell as is seen besides the last whorl, and to describe it as large or small in diameter, compared with the whole: or to say that the whorls of the spire are half, or one-third, or one-fourth covered, as the case may be.

Aperture of Symmetrical Univalves.

In Ammonites Blagdeni and some others the aperture is of an oblong square; it is then said to be sub-quadrated; in Nautilus triangularis it is angulated; in Ammonites Greenoughi it is of an interrupted oval shape, described as elliptical. In the greater number of Orthocerata, it is rounded or circular. The entrance of the last whorl into the aperture of some rounded species of Nautilus causes it to take a semi-lunar form; if rounded at the sides it is said to be reniform or kidney-shaped; if pointed at the sides it is semi-lunar; and in some species of Ammonites, it is five-sided or quinque-lateral.

Measurement of Symmetrical Conical Univalves.

The width is measured across the aperture, which is the widest part of the shell. The length (l) from the dorsal part (d) of the aperture to the dorsal part of the whorl (d) on the opposite part of the shell. The ventral part of the whorls is that nearest to the axis, and the dorsal that which forms the outline of the figure.

NON-SYMMETRICAL UNIVALVES.

These are conical, irregular, spiral, or convolute. The conical form is when there is no enrolment of the apex. Although the Patellæ were described as symmetrical, there are several species of Patelliform shells which are not symmetrical. In Umbrella, for instance, the apex is oblique, the shells being placed obliquely on the animal. In the genus Siphonaria, there is a groove on one side, where the brachia or gills of the animal rest. In the genus Ancylus, it will be observed that the apex bends on one side, and the animal is like the Limnæa, which has a spiral shell. The cup and saucer Limpets, or Calyptrædæ, present a group which requires to be described, differently from the symmetrical or true Limpets. Their structure is very curious, and they vary considerably among themselves, some of them being simply conical, others nearly flat, or discoidal, and others more or less spiral. But their principal peculiarity consists in their having a small internal process or plate variously shaped, commonly named their septum.

Septa of Limpets.

The septa of Limpets assume a variety of forms, the principal of which will be seen in the accompanying engravings.

The form from which the group derives its generic appellation is that of the cup-shaped or Cyathiform species (fig. 17). In the Crepidulæ, or Slipper-Limpets, the septum is flat, reaching across the opening, like the deck of a vessel; it is then described as transverse (fig. 20). In Calyptræa Equestris, it has two prominent points, and is described as bi-furcated (fig. 18). In another species, it is a three-sided plate rather spiral at the apex (fig. 19).

Measurement of Cup and Saucer Limpets.

The line marked a, p, ll indicates the direction in which the shell is to be measured for length. a indicates the anterior, p the posterior. The line d (fig. 23), from the apex to the base, is the depth. The line b (fig. 28), is in the direction of the breadth.

Irregular non-symmetrical Univalves.

Serpuliform shells are irregularly twisted (tortuous) hollow tubes, which were formerly considered to have been secreted by a kind of worm, but now known to be the shells of true Molluscs, of a kind not very widely differing from those which have regularly spiral shells. The greater part of these are attached to foreign bodies, or to each other in groups. Some are attached by the whole length of the shell, they are then said to be decumbent. Some of these are coiled round like the Spirorbis, the little white shell seen on the carapace of the Lobster or on leaves of sea-weeds; they are then said to be discoidal; others again, such as the Vermetus, approach more nearly to the spiral form. The deviation from the regular 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.

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.

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.

Suture.

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.