142 MOLLUSC A. LAMELLIBRANCHIATA. 



impression deep, the anterior much smaller than the 

 posterior, and placed near the umbones. Pedal impres- 

 sions in front of the posterior adductor and one in the 

 umbone of the left valve. Pallial line simple. Interior 

 nacreous. Lias to present day. 



Unio. Shell oval or elongated, with a thin epidermis. 

 Surface smooth, striated or folded. Umbones more or 

 less anterior, often corroded. Ligament external, elon- 

 gated. In the right valve there are two anterior lateral 

 teeth and a long lamellar posterior-lateral tooth ; in the 

 left valve one anterior-lateral, one cardinal, and two long 

 lamellar posterior lateral teeth. Adductor impressions 

 very deep, especially the anterior. Pallial line simple. 

 Purbeck Beds to present day. Lives in freshwater. 



Anthracosia. Shell elongate, oval, inequilateral. 

 Umbones anterior, not prominent. A strong cardinal 

 tooth in each valve. Ligament external in a linear groove. 

 Muscular impressions deep. Pedal impression above the 

 anterior adductor. Carboniferous and Permian. 



Cardinia. Shell trigonal, oval or oblong, very inequi- 

 lateral, compressed, thick, marked by striae of growth. 

 Umbones small, close together. Ligament external. 

 Cardinal teeth small or obsolete ; in the right valve one 

 anterior- lateral tooth, in the left, one posterior-lateral. 

 Impression of anterior adductor very deep. Pallial line 

 simple. Trias to Lower Oolites. 



OBDEE. SIPHONIDA. 



Animal with siphons. Mantle-lobes more or less 

 united at the margins. Both muscles well developed. 



