General Appearance.—British specimens are usually conical, and have their walls folded, corroded in their upper parts, with the radii not developed, and the sutures more or less obliterated; in this state the orifice of the shell is entire, and very broadly oval: I have seen specimens in this condition from Madeira, Brazil, and the Gulf of Corea. The shells when crowded, are rendered cylindrical, and more or less elongated, with the sutures as viewed externally quite obliterated; the surface rugged and much disintegrated; and the orifice nearly circular: in this extreme condition I have seen specimens from England, from near Genoa, and from the mouth of the Plata: the specimens from these last two localities were of remarkable size, being half an inch in diameter, and rather more than half an inch in height. Again, other British specimens (fig. [1 c]), though not nearly so common, are rather steeply conical, and have not only their sutures distinct, but narrow radii are plainly developed on apparently both sides of the sutures; in this case the orifice is slightly toothed, and is rather elongated: I have seen specimens in this condition, but with their walls rather more deeply folded, from the Cape de Verde and Philippine Archipelago. Other specimens, from some unknown tropical sea, differed only in the walls being but slightly folded, and being so perfectly preserved as to be externally covered with membrane: this latter circumstance gave the specimens a peculiar appearance. In this condition were some specimens (fig. [1 d]) attached to oysters sent to me by Professor Agassiz, from Charlestown; and which differed from all the others that I have seen, in the thinness and smoothness of their compartments, and in the facility with which the compartments separated from each other and from the surface of attachment: hence I have called this form, which, until finding more important differences, I must rank as a variety, fragilis: it has stronger claims than the other varieties to be specifically separated; but I suspect that it has been exposed to some peculiar modifying conditions.
Lastly, we have the variety (fig. [1 b]) excellently figured and described by Poli, under the name of L. depressa: this is much depressed, with the walls thin, not folded, with the surface much corroded, with the sutures very distinct, with the radii not at all or barely developed, but with the alæ largely developed, and marked with lines of growth, resulting from the diametric growth of the shell: the orifice is hexagonal, but broadest towards the carinal end: most of the specimens, but not all, have on the under sides of their compartments rugged pillars depending from beneath the alæ, for the purpose, apparently, of supporting the much depressed shell. This much depressed variety attains a larger basal diameter (but not a greater bulk or internal capacity) than any other variety, namely, sometimes three quarters of an inch. The great peculiarities of this variety result, apparently, from its much depressed form, deeply corroded not folded walls, and considerable diametric growth; from the latter cause the alæ are largely developed; as I can find neither internally nor externally any fixed diagnostic character I have not hesitated to rank this form as a variety. Poli found his specimens mingled with the ordinary [C. stellatus], on the shores of Sicily; and I collected at St. Jago, in the Cape de Verde Archipelago, some specimens nearly as well characterised, also associated with the common variety.
The shell itself is dirty-white or gray, or brown: in some varieties, however, the white is nearly pure: internally the parietes are generally tinted dull purple. The corium of the sack is dark greenish-black, with a white edge to the lips lining the aperture between the opercular valves.
Structure of Shell and Radii, &c.—The under side of the parietes is either quite smooth, or marked with slight, branching, depressed lines; or mamillated; or irregularly studded with large pores. I have already alluded to the pillars, depending from the under sides of the alæ in most specimens of var. depressus: these pillars tend to form ridges, parallel to the sides of the compartment, like those we shall presently see in certain specimens of [C. scabrosus]. The radii are very narrow, when best developed; their edges, when disarticulated, can be seen, when examined by a strong lens, to be finely crenated. The edges of the alæ are likewise very finely crenated.
Scuta.—The outline of these valves varies considerably in specimens taken out of the same group: we have either a nearly equilateral triangle (fig. [1 f]) or the tergal margin ([1 e]) is much shorter than the other margins. There is always a deep depression for the adductor muscle, and a small pit of very variable depth for the lateral depressor. But the tergal margin offers the greatest variability; here we see a very prominent articular ridge or fold, having either a straight edge or a single or double prominence (fig. [1 e]-[1 h]). In specimens in the same group we find considerable variation in these points; but the amount of difference is sometimes so great, that I long hesitated whether to rank some of the varieties as species. The Terga, likewise, vary greatly in shape and width: in some of the commonest varieties (and in var. depressus) the valve is very narrow, with the under surface channelled or concave: in other varieties the valve is much broader and flatter. The spur is but slightly developed. The crests for the depressor muscles barely descend beneath the basal margin of the valve. The articular ridge, in some varieties (as in specimens from Madeira and the Cape de Verde Islands), is extremely prominent and straight ([1 f]); in others, it is little prominent and deeply sinuous ([1 h]). This great variability in the articular margins of the scutum and tergum seems to be mainly due to the corrosion to which these valves have been subjected, and their consequently modified growth: in some specimens the articular ridge of the scutum, and in others that of the tergum, has been largely developed, in either case their mutual outlines being greatly affected.
Branchiæ.—These are narrow, hardly at all plicated, elongated, being about half as long as the sack. In a specimen from La Plata, this organ ended rather more abruptly in a point than it did in other specimens.
Mouth.—The crest of the labrum is usually hairy, but in a specimen from Bahia (Brazil) there were some very fine teeth. The palpi vary somewhat in shape, and sometimes have a row of bristles along their basal exterior margins. The mandibles usually have four main teeth, the lowest one being confluent with the inferior pectinated angle: in var. depressus, in the same individual, there were only three teeth on one side of the mouth and five on the other; the lower main teeth are laterally double, but generally one tooth of each pair is so small and obscure as to be perceived with difficulty. In the maxillæ, there are some large spines above the notch, and in the notch some fine ones: in var. depressus, and in a cylindrical var. from La Plata, there was quite a tuft of small spines above the notch.
Cirri.—The outer surface of the pedicel of the second cirrus bears a tuft of long, fine, plumose hairs: the terminal segments of the rami of this cirrus sometimes (as in the La Plata specimens) support a clump of coarsely pectinated spines. In specimens having six segments in the shorter ramus of the second cirrus, the shorter ramus of the third cirrus had fifteen segments. The two rami of the third cirrus are usually equal in length and in the number of their segments; but in the Brazilian specimen there were fifteen segments in the posterior, and twenty-six in the anterior ramus; in another specimen, fixed on a tropical Perna, there were in the two rami of this third cirrus eighteen and twenty-four segments. In the three posterior pairs of cirri each segment carries either four or five pairs of main spines: the segments vary a little in the degree to which they are elongated, being most elongated in the var. from La Plata, with an elongated shell.
Varieties.—It will have been observed, that the shell, in the specimens from several distant quarters of the world over which this species, as I believe, ranges, differs considerably in external aspect: so do the opercular valves; and so do the parts of the mouth and cirri: but I cannot make out that these differences are coordinated. Thus, var. depressus, which is so entirely different from the others in appearance, differs only internally in the presence of a tuft of fine spines above the notch of the maxillæ; and this character is found in the La Plata variety, which, as far as the shell and opercular valves are concerned, is at the other end of the scale of variation. Again, var. fragilis, from Charlestown, presents, in the animal’s body, hardly any difference. The Brazilian specimens, which in the shell and operculum offer only quite common characters, have the remarkable peculiarity of a considerable difference in the length and number of the segments in the rami of the third cirrus; they, also, have the segments of the sixth cirrus considerably elongated, and the labrum finely toothed. Of these peculiarities one alone, namely, the inequality in the rami of the third cirrus, but in a lesser degree, is common to the specimens adhering to the tropical Perna, which had a shell very unlike the Brazilian variety, but which, on the other hand, differed scarcely in a single character from some other specimens from an unknown tropical sea, in which the rami of the third cirrus were quite equal. The La Plata specimens differ most in internal characters, viz., in the tuft of fine spines above the notch of the maxillæ, in the coarsely pectinated spines on the tips of the second pair of cirri, in having the segments of the sixth cirrus much elongated, and in the apex of the branchiæ being abruptly pointed; yet in the shell and operculum they were identical with certain Mediterranean varieties. From these several facts, I must believe that all the widely distributed forms here grouped together, do really belong to the same species.