492 L AMELLIBRANCHI ATA. 



(Eocene). Some forms as old as the Upper Devonian have, 

 however, been referred to this genus. 



Carbonicola, of the Devonian and Carboniferous, comprises 

 Unionoid Bivalves, with thick shells, an external ligament, 

 and a concentrically -striated surface. The beaks are not 

 eroded ; and the hinge has a thick cardinal tooth in the 

 right valve, with a long lamellar lateral tooth on each side. 

 The Devonian and Carboniferous genus Anthracosia is allied 

 to the preceding, but there is a single cardinal tooth in each 

 valve, and lateral teeth are wanting. 



SECTION B. SIPHONIDA. 



Subdivision L Integropallialia. Siphons short, pallial line 

 simple. 



FAM. *7. CHAMID^:. Shell inequivalve, attached. Hinge- 

 teeth 2-1 (two in one valve and one in the other). Impres- 

 sions of the adductors large. Mantle closed; pedal and 

 siphonal orifices small and nearly equal. Foot very small. 

 The most important fossil forms of this family belong (if we 

 exclude Caprina and its allies) to the genera Chama, Diceras, 

 and Eequienia. 



In the genus Chama (fig. 357) the shell is attached 

 usually by the beak of the left valve, but sometimes by 

 that of the right. The upper valve 

 is the smallest, and both bear folia- 

 ceous expansions. The free valve 

 carries one tooth which articulates 

 with two teeth in the attached valve. 

 The CJiamas do not appear as fossils 

 till we reach the Cretaceous rocks, and 

 they have continued to exist up to the 

 present day. 

 Fig. 357.-mterior of the In tne remarkable genus Diceras 



attached valve of CJiama la- /fig. 358), the shell is " Sub-equivalve, 

 mellosa. Eocene Tertiary. \ 



attached by either umbo ; beaks very 



prominent, spiral, furrowed externally by ligamental grooves ; 

 hinge very thick, teeth 2-1, prominent; muscular impres- 

 sions bounded by long spiral ridges, sometimes obsolete " 



