80 GENUS XV, ANOMIA. 



ments close together ; or, if one, it is at least divided into three 

 parts. In some species the flat valve is perforated with three 

 holes, as in A. craniolaris ; whilst in some others the beak of 

 the convex valve is much produced, having the tip perforated, 

 as in A. Caput serpentis. Withinside some of the species there 

 is found a kind of bone, bent or twisted in a curious manner, 

 which serves for a support to the animal, as in A. Terehratula. 

 There are other species in Avhicli both valves are flat, and nearly 

 circular, without any perforation, so that they can never shut 

 close at the circumference or margin, except merely at the 

 hinge : the hinge in these is small, with two diverging ridges 

 on the inside of one valve, and two corresponding hollows in 

 the other, as in A. Placenta (/. 46). 



A great many species of fossil shells have been found that 

 are considered as belonging to this genus. The animals in- 

 habiting these shells are very little known ; but several of them 

 appear to vary mvich from the others, and also from all other 

 genera. Some of the animals have neither tube nor foot that 

 they can protrude beyond the shell, but have two long arms 

 to assist in swimming, ciliated on one side their whole length, 

 which they roll up spirally within the shell when not in use, 

 as in A. Terehratula and vitrea. The animal of A. tridentata 

 has two flat arms somewhat scythe-shaped, and three-lobed, 

 with which he swims in the sea. 



Linnasus describes 27 species, Gmelin has added 24, and 

 Dr. Turton none. Total, 51. But several of the species are 

 fossil, and have not yet been found in a recent state. 



