224 MOLLUSCA. LAMELLIBRANCHIA 



Thetironia ( = Thetis). Shell thin, oval, rounded, very 

 convex, slightly or moderately inequilateral. Umbones prominent, 

 curved inward and slightly forward. No lunule. Ligament 

 external. Two small conical or tubercular cardinal teeth under the 

 umbo in each valve ; no laterals. Adductor impressions near the 

 anterior and posterior margins. Pallial line simple. Two internal 

 ribs meet at an acute angle near the umbo and extend ventrally to 

 the level of the adductors. Surface of shell nearly smooth, with 

 concentric lines and radial rows of small pits which are more 

 distinct on the posterior part than elsewhere. Cretaceous. Ex. 

 T. minor, Lower Greensand. 



(b) Pallial line usually with a sinus, but sometimes 

 sinuous only. 



Venus. Shell thick, oval, convex, ornamented with concentric 

 lamellsG, sometimes with radial ribs ; lunule distinct. Margins of 

 valves finely crenulate. Hinge-plate wide ; in each valve three 

 cardinal teeth, often bifid, no lateral teeth. Ligament external, 

 prominent. Pallial sinus short, angular. Miocene to present day. 

 Ex. V. casina, Pliocene to present day ; V. verrucosa, Eecent. 



Meretrix (fig. 92). Shell thick, ovate, sub-trigonal, convex, 

 smooth or with concentric ornament. Margins of valves smooth. 

 Lunule present. Ligament external. Hinge-plate thick, with three 

 cardinal teeth in each valve, two anterior laterals in the right, and 

 one in the left valve. Pallial sinus angular or rounded. Cretaceous 

 to present day. M. meretrix, Recent ; Ex. M. {Callista) planus, 

 Upper Greensand ; M. {Callista) chione, Recent. Meretrix is here used 

 in a wide sense, and includes Callista, Cytherea, Tivela, Pitaria, etc. 



Dosinia ( = Artemis). Shell orbicular, compressed, with con- 

 centric ridges or 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. Oligocene to present 

 day. Ex. D. exoleta, Coralline Crag to present day. 



Tellina. Shell oval, elongate, sometimes sub -orbicular, 

 slightly inequivalve, compressed, rounded in front, attenuated 

 behind, and furnished with an oblique fold from the umbo to the 



