402 TOOTH-SHELLS. 



resemblance to chalcedony. The substance of the cup is very delicate, hardly thicker 

 than the paper on which this account is printed. 



ANOTHEK species of the same genus, the LADY'S BONNET, is seen on the same 

 engraving. The generic name is derived from the word calyptra, which signifies a lady's 

 cap. The food of these molluscs seems to be rather varied, as they are known to eat the 

 minute algae, and one specimen has been observed in the act of devouring a little sea-slug 

 which was placed in the same vessel. 



It may be here remarked, that all the Limpets seem to alter the shape of their shell to 

 suit the surface to which they adhere ; and even the common Limpet of our own shores 

 will be frequently found to have modified the edge of its shell into very curious outlines, 

 for the purpose of accommodating it to certain hard projections or depressions of the 

 rock to which it clings. As these molluscs possess and exercise the power of loco- 

 motion, it is remarkable that they should mould their habitations into eccentric forms 

 that can only suit one particular spot. For example, if a Limpet be attached to a 

 concave surface, the shell becomes convex in a precisely corresponding degree ; should 

 it prefer a convex body, the shell is concave; and in many cases where the shell is 

 affixed to a cylindrical object; such as a submerged stick, or perchance the stem of a 

 large seaweed, it adapts itself so thoroughly to its support, that it clasps the cylinder in a 

 partial embrace. It is possible that the power of adaptability may be rapid in its 

 operation, and that the animal can supply new matter to the shell when needed, so as to 

 render it concave or convex at pleasure, or to fill up the grooves and notches which have 

 existed when it clung to an irregular surface. 



Two specimens of the HTJNGAEIAN BONNET LIMPET are given in the same illustration. 



This shell is almost invariably found adhering to oysters in a moderate depth of 

 water, varying from five to fifteen fathoms, though it sometimes prefers a greater depth. 

 The finest specimens are, however, taken in the shallower waters. The popular name 

 is sufficiently appropriate in this instance, as the shell is exceedingly like the celebrated 

 Phrygian bonnet of the ancients, or the republican cap of a later period. 



The COMMON LIMPET is so familiar that it need not be figured nor described. One 

 species of its genus attains to an enormous size, measuring a foot in diameter, and having 

 a shell of very great thickness. 



THE next family, called appropriately Pentalidre, or the Tooth-shells, have long 

 puzzled zoologists to assign their right position in the scale of nature, and even baffled 

 the wide experience and penetrative acuteness of Cuvier himself. The general opinion of 

 the systematic naturalists of his time referred the Dentalidse to the annelids or worms ; 

 but Cuvier always expressed his doubts as to the accuracy of their views, and remarked 

 that the solution of the problem would be found in the nervous and respiratory 

 systems. 



This conjecture has been followed up by later anatomists, and the Dentalida? have 

 not only been recognised as molluscs, but have actually been referred to their proper 

 position in that order. Let one of the perforated shells, such as the fissurella, be 

 enormously lengthened in proportion to its width drawn out, in fact, like glass under the 

 hand of a workman and it will form a very good idea of the Dentalium. The animal 

 must necessarily undergo certain modifications to enable it to accommodate itself to the 

 elongated habitation, and in consequence is so long and worm-like as to have induced 

 many experienced naturalists to refer it to a class very far below that which it rightly 

 occupies. 



In all these creatures the shell is formed very much like an elephant's tusk, except 

 that a small perforation exists at the small end. The animal is very curiously formed, 

 being remarkably worm-shaped, and having a conical foot with a- rather sharp point, and 

 holes at the side. The animal is attached to the shell close to the little orifice at the 

 point, and has no definite head, no tentacles, and no eyes. The creature seems to be 

 almost wholly carnivorous. 



