MOLLUSCA. LAMELLIBRANCHIA 225 



posterior border. Margins of valves smooth. Two cardinal teeth in 

 each valve, and one anterior and one posterior lateral which are 

 often indistinct in the left valve. Ligament external, prominent. 

 Pallial sinus very deep. Jurassic to present day. Ex. T. virgata, 

 Recent ; T. rostralis, Eocene. 



Psammobia ( = Gari). Shell elongate, sub-equilateral, gaping 

 at the ends, anterior side rounded, posterior side more or less 

 truncate and angular. Surface smooth or with stria?. Ligament 

 external, thick, joined to prominent ridges. Usually two cardinal 

 teeth in each valve, some being bifid. Adductor impressions near 

 the dorsal border. Pallial sinus very deep. Eocene (perhaps 

 Cretaceous) to present day. Ex. P. ferroensis, Coralline Crag to 

 present day. 



Solen. Shell very long, sub-cylindrical, straight, smooth or 

 finely .striated, the dorsal and ventral margins parallel \ gaping at 

 both extremities. Margins of valves smooth. Umbones at the 

 anterior end. Hinge terminal, with one cardinal tooth in each valve. 

 Ligament long, external. Anterior adductor impression elongated, 

 parallel to the dorsal margin. Pallial sinus short. Eocene (perhaps 

 earlier) to present day. Ex. S. obliquus, Bracklesham Beds ; >S. 

 vagina, Recent. 



Mactra. Shell oval or trigonal, nearly equilateral, smooth or 

 with concentric striae. Internal ligament in a large triangular pit. 

 External ligament in a groove. In front of the internal ligament- 

 pit is a bifid cardinal tooth (in the form of an inverted V) ; anterior 

 and posterior lateral teeth well-marked, compressed, double in the 

 right valve, single in the left. Adductor impressions semicircular. 

 Pallial sinus round or angular. Cretaceous to present day. Ex. 

 31. ovalis, Red Crag to present day. 



My a (fig. 91). Shell oblong, gaping at both ends, particularly 

 at the posterior ; the left valve a little smaller than the right. In 

 the right valve a very small cardinal tooth ; in the left valve a large 

 spoon-like process to which the internal ligament is fixed. Anterior 

 adductor impression elongated. Pallial sinus large and rounded. 

 Eocene to present day. Ex. M. truncata, Pliocene to present day. 



Corbula. Shell oval, inequi valve, closed, rounded in front, 

 somewhat angular and contracted behind, with a ridge passing from 



w. p. 15 



