EXTERNAL FORM OF SHELLS. 413 



water. But besides these entire genera, the shells of which 

 are invariably sinistral, there are numerous species of Buli- 

 mus, Partula, Pupa, and Chondrus, that are uniformly so 

 twisted ; and there are even some that are sometimes twisted 

 in one direction and sometimes in the other, as Bulimus 

 lyonettianus, B. aureus, &c. Among the marine shells the 

 sinistral direction is much more rare, although there are a 

 few species, such as Fusus sinistrorsus of Deshayes, P. con- 

 trarius and P. sinistratus of Lamarck, and some species of 

 Cerithia, which are constantly so contorted. The Pyrula 

 perversa is as often found twisted in one direction as in the 

 other, and its shells have even been considered as different 

 species according to the direction of their whorls. Of other 

 marine species liable to the same variation, I have observed 

 Buccinum undatum, which is not uncommonly found re- 

 versed ; Turbinella napus, the reversed variety of which is 

 much sought after by the Chinese ; Oliva oryza, Nassa reti- 

 culata and N. thersites ; but there are some genera in which 

 I do not recollect to have noticed its occurrence ; as, for 

 example, the Cypraeas and Ovulae. 



A bivalve shell is composed of a dextral and a sinistral 

 valve, united together by a ligament. When the two valves 

 are separated, and spread out on a table with the umbones 

 above, and the front end towards the observer, the valve to 

 the right (the left when on the animal and in its usual walk- 

 ing position) resembles a dextral, and that to the left a sinis- 

 tral, very depressed spiral shell. This is well illustrated by 

 comparing the left valve of an Isocardia with a Concholepas. 

 In some very rare instances these shells are also reversed, 

 but the fact is not easily observed, except in the unequal- 

 valved kinds. There were formerly in the Tankerville col- 

 lection two specimens of Lucina childreni, in one of which 

 the right valve was a dextral shell, in opposition to the 

 general structure. A much more remarkable variation is to 

 be observed in some of those bivalve shells whose under 

 valve is attached to foreign bodies ; thus, for example, most 

 of the Chamae are attached by their left valve, but some 

 species, such as Chama lazarus, are frequently attached by 

 their right valve, under which circumstance the teeth proper 

 to the left and usually attached valve are transferred to the 

 right, and vice versa. 



The equality or inequality of the valves of bivalves appears 

 to be dependent on the habitual position of the animal. 

 Thus all the genera whose animals bore perpendicular holes 

 in rocks, like the Pholadcs ; or bury themselves in the mud 

 of rivers, as the Uniones ; or in the sand of the sea-coast, as 



