402 RELATION OF SHELL AND ANIMAL. 



mantle in the common cockle, which is regularly raised and 

 depressed at short intervals, corresponding to the grooves 

 and ribs of its shell. But very often these fleshy appen- 

 dages of the mantle appear to be developed only at certain 

 periods, and, after the orgasm is over, again to fall and 

 subside into inactivity. In their times of development 

 cases of calcareous matter are formed for their protection, 

 and, before the animal proceeds to add a new piece to its 

 domicile, it fills up these cases more or less completely with 

 shelly matter, which then remain, in the form of cross- 

 ridges, to constitute the shell's principal feature and beauty. 

 In this manner are formed the vaulted spines on many 

 Cockles ; the strap-like processes of the Spondyle ; the thick 

 ribs that gird many Rock-shells ; the spinous, pectinated and 

 foliated processes of many other Muricidae ; and, in short, all 

 the irregularities and inequalities which appear, at inter- 

 rupted intervals, on the surface of every shell. 



But although, from the preceding remarks, you might 

 expect to find an index to the animal's organization in the 

 exterior aspect and model of the shell, yet you must consult 

 the index with heedful caution, for if solely depended upon, 

 it will occasionally mislead you. This is now admitted. 

 Montagu affirms, " that similar shells are sometimes inha- 

 bited by very different animals." * Cuvier has frequently 

 insisted on this fact ; and Mr. Collier goes so far as to say, 

 that " many Mollusca, alike in form and structure, inhabit 

 shells so essentially different in character, as to render the 

 union of the two modes of distinction impossible." f In a 

 book which happens to lie on my table, I find an apposite 

 example in a new species of Mollusk, found by Mr. Alder at 

 Dalkey Island, near Dublin. The shell was in all respects 

 similar to that of the genus Rissoa, but the snail had four, 

 and not two, tentacula ; and the eyes were placed on the 

 back at some distance behind the tentacula, and not at their 

 base. J This subject, which is one of considerable import- 

 ance to the geologist, has been handled so ably by Mr. J. 

 E. Gray, that I cheerfully avail myself of his permission to 

 send you his remarks, unmutilated. 



* Test. Brit. pref. iii. f Edinb. New Phil. Journ. vii. 225. 



J Reports Brit. Assoc. 1843, Trans, p. 74. 



