SIPHONIDA. 



505 



FAM. 16. TELLINID^E. Shell free, usually equivalve and 

 closed ; cardinal teeth two at most, laterals 1-1, sometimes 



Fig. 371. A, Hinge of Mactra solida Recent ; B, Interior of right valve of Donax retvsa 

 Eocene ; c, Interior of right valve of Psammobia rudis Eocene ; D, Interior of right valve of 

 Cultellus polita Recent. 



wanting. Ligament on the shortest side of the shell, some- 

 times internal. Mantle widely open in front. Siphons long 

 and slender ; foot tongue-shaped, compressed. Pallial sinus 

 very large. The chief fossil genera are Tellina, Psammobia, 

 and Donax. In Tellina 

 the shell is very slightly 

 inequivalve (fig. 372) with 

 a prominent external liga- 

 ment. More than a hun- 

 dred fossil species are 

 known, dating from the 

 Oolitic period ; but the 



genus has attained its maximum at the present day. In 

 Psammobia (fig. 371, c) the shell is oblong, compressed, and 

 slightly gaping at both ends; whilst in Donax (fig. 371, B) 

 the shell is wedge-shaped, the front rounded and produced, 

 the posterior side short. Both genera commence in the 

 Eocene Tertiary, and are represented by numerous species at 

 the present day. 



Among the remaining types of the Tellinidce, the genera 

 Soiverlyia and Quenstedtia are Jurassic, and we may perhaps 

 place here the Tancredia of the same formation, which much 

 resembles Donax in general form, but is said to possess an 



Fig. 372. Tellina proximo,, right valve. 

 Post-Pliocene. 



