EXTERNAL FORM OF SHELLS. 411 



may also be observed on the shelly plates of the Cirrhipedes. 

 In some instances, as in the Cowries and Melons, the outer 

 coat of the shell is covered in the adult age with a deposi- 

 tion of shelly matter, which entirely conceals it from view. 

 A few shells, as for example those of Loligo and Aplysia, 

 contain so little calcareous matter, as to appear to be formed 

 entirely of periostracum. 



2. The External Form of Shells, and their Variations. 



Each valve of a shell, according to the manner in which 

 it is first formed and subsequently increases, is a more or 

 less depressed or lengthened cone. The apex of this cone 

 is always oblique : in all the shells with which I am ac- 

 quainted, it is excentric ; and in most of the univalve shells, 

 whether they be simply conical, involute or spiral, it is 

 directed from the head of the animal towards the hinder 

 part. The only exception, as far as I know, to this rule, 

 occurs in the genera Patella and Lottia, in which the apex 

 is directed from the hinder part towards the head ; and this 

 is the more remarkable, as in the Chitons, the animal of 

 which so much resembles that of Patella, each of the valves 

 takes the usual direction. The similarity of direction in 

 the two genera above named is still more curious, as their 

 animals bear scarcely any resemblance to each other. 



The nucleus of the bone of the Cuttle-fish and of the 

 Loligo, is placed in the same direction ; for it is the conical 

 process at the end of the bone of the Cuttle-fish (called 

 Beloptera, when found in the fossil state), which must be 

 regarded as the nucleus of these shells. If, however, the 

 relative position of the animal of the Nautilus, the anatomy 

 of which has been admirably described by Mr. Owen, be 

 correctly assigned by that author with respect to its shell, 

 it must offer a similar anomaly with the genera Patella and 

 Lottia. The shells of the Pteropods, as for example Hya- 

 lsea, Cleodora and Vaginella, take the same direction as the 

 other univalves ; and it was this circumstance that gave 

 rise to the supposition that M. de Blainville, in his figure 

 of the animal of Cymbulia, had placed it in the shell in the 

 wrong position. The numerous specimens which are now 

 in European cabinets have proved the accuracy of this sup- 

 position. 



In bivalve shells the apex of each valve is always placed 

 on or near the dorsal or hinge margin, varying its position 

 on this part in the different groups. Thus, in the Pectines 

 and other suborbicular shells, which having a very large 



