“C. tuberculatum,” says Mr. Gosse (who described it from specimens which I sent him in 1854), “is far the finest species. The valves are more globose and of a warmer colour; those that I have seen are even more spinous.” Such may have been the case in those I sent: but it has occurred to me now and then to dredge specimens of C. aculeatum, which had escaped that rolling on the sand fatal in old age to its delicate spines, and which equalled in colour, size, and perfectness the noble one figured in poor dear old Dr. Turton’s “British Bivalves.” Besides, aculeatum is a far thinner and more delicate shell. And a third species, C. echinatum, with curves more graceful and continuous, is to be found now and then with the two former. In it, each point, instead of degenerating into a knot, as in tuberculatum, or developing from delicate flat briar-prickles into long straight thorns, as in aculeatum, is close-set to its fellow, and curved at the point transversely to the shell, the whole being thus horrid with hundreds of strong tenterhooks, making his castle impregnable to the raveners of the deep. For we can hardly doubt that these prickles are meant as weapons of defence, without which so savoury a morsel as the mollusc within (cooked and eaten largely on some parts of our south coast) would be a staple article of food for sea-beasts of prey. And it is noteworthy, first, that the defensive thorns which are permanent on the two thinner species, aculeatum and echinatum, disappear altogether on the thicker one, tuberculatum, as old age gives him a solid and heavy globose shell; and next, that he too, while young and tender, and liable therefore to be bored through by whelks and such murderous univalves, does actually possess the same briar-prickles, which his thinner cousins keep throughout life. Nevertheless, prickles, in all three species, are, as far as we can see, useless in Torbay, where no wolf-fish (Anarrhichas lupus) or other owner of shell-crushing jaws wanders, terrible to lobster and to cockle. Originally intended, as we suppose, to face the strong-toothed monsters of the Mediterranean, these foreigners have wandered northward to shores where their armour is not now needed; and yet centuries of idleness and security have not been able to persuade them to lay it by. This—if my explanation is the right one—is but one more case among hundreds in which peculiarities, useful doubtless to their original possessors, remain, though now useless, in their descendants. Just so does the tame ram inherit the now superfluous horns of his primeval wild ancestors, though he fights now—if he fights at all—not with his horns, but with his forehead.