730 



LAMELLIBRANCHIATA. 



going currents of water ; and the foot is rudimentary. The shell is 

 irregular, very inequivalve, always attached to foreign objects by the 

 substance of one valve. In structure, the shell is thick, and consists 

 of an internal porcellanous and an external prismatic layer, often 

 with special and remarkable modifications in particular types. The 

 hinge is thick, and is provided with cardinal teeth, but lateral teeth 

 are wanting. The ligament may be external or internal, or is want- 

 ing altogether. The pallial line is entire, and the two adductors 

 leave well-marked scars, or may be inserted upon special internal 

 calcareous septa. This order includes the two families of the 

 Chamidce and the Rudistce. 



Family i. Chamidce. — In this family the shell is inequivalve, 

 and is generally attached to foreign objects by the substance of one 

 of the valves. The ligament is normally external, and lies in a deep 

 groove, but it may become more or less completely concealed from 

 view. The hinge is massive, and usually carries two teeth in one 

 valve and one in the other. The adductor impressions are very 

 large, and the pallial line is entire. The family includes the single 

 recent genus Chama, together with a number of exceedingly remark- 

 able Mesozoic Bivalves, such as Diceras, Requienia, Monopleura, 

 Caprina, Cafirotina, &c. The structure of these latter is, however, 

 so complicated that they cannot be advantageously treated of here 

 except in an exceedingly brief manner ; since a full understanding 

 of their characters can only be obtained by the examination of 

 actual specimens. 



In the genus Chama (fig. 607) the shell is attached to foreign 

 objects by the beak of the left valve, or, more rarely, by that of 

 the right valve. The beaks of both valves are incurved and are 

 directed anteriorly. The free valve is the 

 smallest, and the surface of both valves is 

 furnished with foliaceous expansions, derived 

 from a thick external prismatic layer, below 

 which is situated an internal porcellanous 

 layer. The massive hinge usually carries a 

 single tooth in the free valve, articulating 

 with two teeth in the attached valve. The 

 recent species of Chci7na are all inhabitants 

 of the sea. The fossil species commence 

 in the Cretaceous rocks, a number of forms 

 being found in the Tertiary deposits. 



In the genus Requienia (fig. 608) the shell 

 is exceedingly inequivalve, the right valve 

 being small, subspiral, and operculiform, while the left valve is of 

 large size and is spirally rolled up, its beak serving for the attach- 

 ment of the shell to some foreign body. The ligament is external, 



Fig. 607. — Interior of the 

 attached valve of Chcnna la- 

 mellosa. Eocene Tertiary. 



