Shell conical, depressed, heavy: radii rather narrow, depressed, generally notched on both sides.
Hab.—Mediterranean; west coast of Africa; north-east coast of Australia; Low Archipelago, Pacific Ocean; west coast of Mexico, 23° N. Attached to turtles.
It is impossible to feel sure which of the three species of the genus Linnæus had in view, when describing his Lepas testudinaria; but as Spengler has well discriminated the following species under the specific name of caretta,[123] and Ranzani the third species under patula, the present name may, without question, be retained for the following species. In several respects this species is intermediate between [C. caretta] and [patula], but it can most conveniently be described first.
[123] I am indebted to Dr. J. E. Gray for having guided me to this identification.
General Appearance.—Shell strong, globulo-conical, depressed; outline broadly oval; surface smooth, generally well preserved, but when disintegrated, upper part finely striated; colour dead white; orifice oval elongated, rather exceeding in length one third of the longer basal diameter of the shell. The radii are rather narrow, and deeply depressed; they have their summits square: their outer lamina, as explained under the genus, on both sides of each suture is in most specimens divided into teeth, the points of which face and touch each other. These teeth or notches give quite a peculiar appearance to the shell, and alone suffice to discriminate this species; they are sometimes blunt and partially obliterated, but it is rare to find a specimen in which some few teeth do not occur in some one of the six radii. I have, however, seen two or three specimens with all six radii perfectly smooth; in one of these the general shape of the shell, without the aid of any internal characters, almost sufficed to show that it belonged to the present species; but in another specimen, which had unusually narrow radii, and the whole surface of which had undergone considerable disintegration, and was consequently striated, could only be distinguished from [C. caretta] by internal characters. I have seen several specimens having very irregularly shaped compartments, but generally the appearance of the whole shell is highly symmetrical, like a star; and the genus was appropriately named by old Klein, Astrolepas. In some specimens, in Mr. Cuming’s collection, from the Low Archipelago, in the Pacific, taken off the toe-nail of a turtle, the shape was almost cylindrical; the shell almost resembling that of [Coronula diadema]. The largest specimen which I have seen was 2.3 of an inch in basal diameter, but only .55 in height.
Structure of Shell and Radii.—After the full generic description, the only point to which I need allude is, that the radiating, parietal septa, as well as the descending sheath, are much thicker than in [C. patula], and that their basal edges can be plainly seen by the naked eye to be dentated with numerous points. The thickness of these plates and of the sheath varies considerably. In [C. caretta], I may add, the plates are broken up into many separate points, and in this species the descending sheath is not generally perforated, excepting at the sutures, by loop-holes for the entrance of ribbons of corium. The sheath and inner lamina of the parietes in [C. testudinaria], taken together, fully equal one third of the entire thickness of the walls. Flattened cavities or tubes run up between the parietal septa for about two thirds of the height of the inside of the shell. With respect to the radii, I have only to add, that the thickness of the inner portion relatively to the generally notched outer lamina, varies considerably.
Opercular Valves (fig. [1 b]-[1 d]).—These closely resemble each other in all three species, and have been almost sufficiently described under the genus. The scutum has its occludent margin always sinuous, but to a variable degree. In one specimen there was a deep little pit at the rostral end for the rostral depressor muscle. The terga vary considerably in shape, according as the basi-carinal corner is more or less truncated. The external furrow, marking the position of the rudimentary spur, varies much in distinctness, chiefly depending upon its nearness to the carinal margin of the valve.
Mouth and Cirri.—The labrum sometimes has its teeth worn away. The second pair of cirri, relatively to the others, is shorter and thicker than in the two following species.