146 MOLLUSCA. LAMELLIBRANCHIATA. 



Section 2. Sinupalliata. 



Siphons long, partly or completely retractile. Pallial 

 line with a sinus. 



Venus. Shell thick, oval, inflated; surface smooth or 

 with ribs or concentric lamellae ; lunule distinct. Margin 

 of valves finely crenulate. Hinge-plate wide with three 

 cardinal teeth, often bifid. Ligament external, prominent. 

 Pallial sinus small. Middle Jurassic to present day. 



Cytherea (=Meretrix). Shell thick, ovate, sub- 

 trigonal, smooth or with concentric furrows. Margin of 

 valves not crenulate. Lunule well-marked. Ligament 

 external. Hinge thick, with three cardinal teeth in each 

 valve, two anterior-lateral in the right, and one in the left 

 valve. Pallial sinus angular. Jurassic to present day. 



Artemis (= Dosinia). Shell orbicular, compressed, 

 with concentric striae. Lunule depressed. Escutcheon 

 narrow. Ligament sunk. Three cardinal teeth in each 

 valve, one anterior-lateral in the left valve, and two 

 (rudimentary) in the right. Margins smooth. Pallial 

 sinus very deep. Cretaceous to present day. 



Tellina. Shell transverse, oval, elongate, or sub- 

 orbicular, slightly inequivalved, compressed, rounded in 

 front, attenuated behind, and furnished with an oblique 

 fold from the umbone to the posterior border. Margin 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, promi- 

 nent. Pallial sinus very deep. Lower Cretaceous to 

 present day. 



