TELLINACEA. 747 



ally obsolete. Little can be said with certainty as to the Palaeozoic 

 shells usually referred to Lucina, but the genus is abundantly repre- 

 sented in Secondary and Tertiary deposits ; and ancient representa- 

 tives of the genus have been described from the Silurian and De- 

 vonian rocks. Cordis (fig. 632, a), with many species from the 

 Trias onwards, is very like Lucina, but has the surface concentri- 

 cally furrowed, with denticulate edges. Diplodonta (Cretaceous to 

 Recent) has two cardinal teeth in each valve, the anterior in the 

 right and the posterior in the left being bifid (fig. 632, b). Lastly, 

 the genus Axinus of Sowerby may perhaps be referred here, though 

 the hinge is toothless or has only a feeble tooth in the right valve. 

 The range of this genus is from the Eocene to the present day. 



Order XIII. Tellinacea. 



In this order the foot is very large ; there are two adductor 

 muscles ; there is only a single gill on each side ; and the siphons 

 are long and completely separate. The shell is free, non-nacreous, 

 the hinge with cardinal and lateral teeth, and the pallial line deeply 

 sinuated. This order includes the two families of the Tellinidce and 

 Scrobiculariidce, all the members of which are marine in habit. 



Family i. Tellinid.e. — In this family the shell is free, usually 

 equivalve and closed, with smooth margins. The hinge has at most 

 two cardinal teeth in each valve, with a lateral tooth on each side, 

 or without lateral teeth. The ligament is external, and the pallial 

 sinus is very deep. 



The principal genus in this family is Tellina itself, which includes 

 about three hundred living species and a considerable number of 

 fossil forms, mostly from the 

 Tertiary rocks. In this genus 

 the shell is oval or transversely 

 elongated, very slightly inequi- 

 valve, the anterior side being 

 rounded, and the posterior side 

 often anarulated. The beaks are 



_ , _ 1 • 1 • t Fi?. 633. — Tellina proxi>na, right valve. 



often placed nearly in the mid- Post-Pliocene. 



die of the shell j the ligament is 



prominent and external ; and the pallial sinus is very broad and 



deep. The oldest undoubted forms of Tellina appear in the Lower 



Cretaceous rocks. 



Family 2. Scrobiculariidce. — In this family the animal agrees 

 with that of Tellina in general structure, and the characters of the 

 shell are in most respects the same ; but the ligament is internal, 

 and is lodged in a pit below the beaks, an external ligament being 

 sometimes imperfectly developed as well. The two principal genera 



