740 



LAMELLIBRANCHTATA. 



shell is oblong, compressed, and nearly equilateral (fig. 621). 

 There are many recent species of this genus, together with a 

 moderate number of Tertiary forms, but the earliest appearance of 

 the genus seems to be in the Cretaceous rocks. The Jurassic 

 genus Quenstedtia appears to be nearly allied to Psammobia. 



Family 10. Solenid^e. — In this family respiratory siphons are 

 present, and are usually short and more or less united, but may be 

 longer and separate. The foot is very large, and more or less 

 cylindrical in shape ; and the branchiae are prolonged into the 

 branchial siphon. The shell is transversely elongated, more or 

 less gaping at both ends, equivalve, and covered with epidermis. 

 The hinge is variable, carrying from one to three cardinal teeth in 

 each valve, without lateral teeth. The ligament is external, and 

 the pallial line is more or less sinuated. The recent members of 

 this family are all marine or estuarine in habit, and the group attains 

 its maximum development at the present day. 



Of the genera of this family, Solecurtus (fig. 622) has an elongated 

 shell with subcentral beaks, the dorsal and ventral margins being 



Fig. 622 



-Solecurtus Deshayesi. 

 (After Zittel.) 



Eocene. 



Fig. 623. — Interior of the right valve of 

 Siliqua polita. Recent. 



nearly parallel, and the surface generally marked with oblique lines. 

 The genus begins in the Cretaceous rocks and still survives. In 

 the genus Siliqua (fig. 623) the shell agrees in form with that of Sole- 

 curtus, and also has the beaks placed a little in front of the centre, 

 but an oblique internal rib runs from below the beak to the ventral 

 margin. This genus ranges in time, as does the preceding, from the 

 Cretaceous to the present day. Cultellus, ranging from the Eocene 

 to the present day, resembles the preceding in most characters, but 

 the beaks are placed very far forwards, and the shell thus becomes 

 very inequilateral. In the genera Ensis and So/en are included the 

 typical " Razor-shells," in which the shell is greatly elongated, with 

 the beaks placed almost at the anterior end of the shell, and with 

 both extremities widely gaping. Ensis begins in the Tertiary rocks 

 and is represented at the present day, its chief distinction from Solen 

 being that the shell is somewhat curved, whereas in the latter it is 

 straight. Solen itself is said to occur in deposits as old as the 

 Devonian, but the earliest undoubted species are found in the 



