Structure of Shell.—The rostrum (fig. [4 c]) is remarkable from its small size, and from the plainness of its sutures, in comparison with those separating the other compartments, and this is exactly the reverse of what I should have expected in a compartment tending to become rudimentary. Not only is the rostrum small, but the alæ project to an unusually small degree, and gradually slope away into the lower part of the parietes. These peculiarities are even more strongly marked in very young shells: thus in one of the size of a pin’s head, the rostrum consisted of a minute parallelogram, without, as far as I could see, any alæ, and was only one fourth of the size of the carina,—this latter compartment being only half as wide as the lateral compartments. The carina, at this early period, had quite distinct alæ.
The tubular prolongations from the four external holes are of course very conspicuous on the under side of the shell ([4 b]); and their structure is there plainly seen. In extremely young shells the holes are not present; but very soon, at four points of the circumference, namely, two in the carina and one in each lateral compartment, the basal edge becomes indented, and during growth more and more deeply indented; at last the horns or points of the bays, thus formed in the circumference of the shell, grow inwards and meet, the four indentations being thus converted into four rings or holes; as the shell is added to, at its circumference, these come to stand further and further from the exterior margin; and as the shell at the same time rises above the surface of attachment, the holes are added to at their basal edges, and are thus converted into shelly tubes, generally freely open at the bottom as well as at the top. Sometimes these tubes are closed at the bottom, and this is usually caused by their sides having been added to in a spiral direction. A somewhat sinuous double ridge or fissure, leading from the tubes or holes to the exterior border, can always be perceived on the under side of the shell. Occasionally, though rarely, in very old shells, a second series of holes is formed outside the first four holes, and often a tendency to this may be perceived in the just-mentioned fissures expanding a little at their outer ends, thus forming four new circumferential indentations. The purpose of this peculiar structure, apparently, is to give support to the much depressed and thin shell. In [Chthamalus stellatus] and [scabrosus], we have seen a slight indication of a similar structure, in the formation on the under side of the shell, but confined to the lines of sutures, of obscurely tubular pillars: we have also something analogous in the singular midribs, in [Platylepas], causing the membranous basis to be convex. I need only further add, that the parietes, in [Chamæsipho], are rather thin, and are composed of translucent shell, punctured for the entrance of tubuli, with the punctures often arranged in straight lines.
Opercular Valves (fig. [4 d]).—These are attached by a narrow opercular membrane to the sheath, but little beneath the summit of the shell. The Scuta are considerably arched or convex: the articular ridge is very prominent, and there is a thick strong adductor ridge. The Terga have a short, rather broad, rounded spur, placed very nearly in the middle of the valve: the crests for the tergal depressores are moderately developed, and are simple.
Mouth.—The crest of the labrum is hairy, and is much hollowed out. The palpi are small and narrow, with long bristles at their apices. The mandibles have four or five teeth, with the lower part pectinated. The maxillæ are notched. Of the Cirri, the second pair is short, and all the segments are thickly clothed with bristles. The third pair in the arrangement of the bristles resembles the four posterior pairs. The segments in these pairs are elongated, and support four pairs of main spines.
I was not able to observe any Branchiæ.
14. PACHYLASMA—Nov. Genus.[133] Pl. [19], [20].
[133] Παχυς [Greek: Pachus] thick, and ελασμα [Greek: elasma], a valve.
CHTHAMALUS. Philippi. Enumeratio Mollusc. Siciliæ.