414 EXTERNAL FORM OF SHELLS. 



the Cardia; or walk freely about on the shores, as the 

 Veneres ; or are attached by a byssus which passes out of a 

 gape formed by the inflection of the margins of both valves, 

 as the Tridacnae, Saxicavae and some Arcae, have equivalve 

 shells : whilst on the other hand, all those Mollusca whose 

 shells are immediately attached by the outside of one of their 

 valves, as the Etheriae, Ostreae, Spondyli, Hinnites and 

 Chamae ; or of which the animals are attached by a byssus 

 passing through a groove near the umbo of one of the valves 

 only, as the Pectines, Aviculae, Peda and Anomiae ; or which 

 lie free on the surface of one of their valves, as the Ostreae, 

 Anatinae, and some of the Arcae, are more or less inequi- 

 valve. In those inequivalve shells which are attached by the 

 intervention of a byssus, this substance passes out through 

 a groove in the right valve, which is the smallest ; whilst, on 

 the other hand, in those that are immediately attached by 

 the outside of the shell, the right valve is affixed, and the 

 left is the smallest, sometimes indeed so disproportionately 

 as to appear like a lid to the other. It is only in the families 

 Ostreidae and Anatinidae, which always lie on their sides, 

 and have unequal valves, that there are found some genera 

 entirely free, and others which are immediately attached. 

 The free inequivalve shells offer some curious anomalies in 

 the relative size of their valves ; nearly all the Anatinidae, 

 as Anatina, Periploma and Magdala, having the left valve 

 the smaller, as have also the genera Corbula and Sphaenia of 

 the family Myidae ; whilst the other two genera of that 

 family, My a and Pandora, and Lyonsia among the Anatinidae, 

 have the right valve the smallest. 



In the Terebratulae and Branchiopodous Mollusca in 

 general, the valves being applied to the dorsal and ventral, 

 instead of the lateral surfaces of the animal, their lateral 

 halves are analogous in situation to the right and left valves 

 of other bivalves, and the byssus by which the animals are 

 attached passing through a hole in the centre of the dorsal 

 valve, the sides of the shell are equal. The dorsal or perfo- 

 rated valve is superior and convex in all the genera of this 

 order, with the exception of Discina, in which, the usual 

 position of the animal being reversed, it is inferior and 

 flattened. 



In all shells, the young of which I have had an oppor- 

 tunity of observing, the nucleus or shell of the animal when 

 first hatched is regular. The irregularity in the form of 

 adult shells appears to depend on their becoming attached to 

 foreign substances, for it is only among attached shells that 

 any irregularity of form is found, and even these are per- 



