248 CONCHYLIA— D/THri?^. 56. 



Besides the general outline, this genus is distinguished by the 

 form and number of the teeth. The primary ones are very minute, 

 consisting of a triangular or cloven tooth with an oblique laminar 

 one in one valve, closing between two plain laminar divergent teeth 

 in the other valve : the lateral teeth are transverse, flat and some- 

 what pointed, placed on each side the hinge, single in one of the 

 valves, and closing between double ones in the other valve. 



* suborbiculares, cequilaterales. 1-3, 

 somewhat orbicular and equilateral. 



rivicola. Cyclas testd suborhiculari tequilaterali convexd opacd, striis trans- 

 versis confertis recjularibus. 

 Shell somewhat orbicular equilateral convex and opake, with regular 

 close-set transverse strise. 



Tab. nost. 11. fig. 13. 

 Mils. nost. From the Thames, and Oxford. 



Shell three quarters of an inch in diameter, covered [with a 

 blackish-brown skin, much more orbicular and regularly striate than 

 the next species, and not so globular : it is sometimes marked with 

 a few transverse paler zones, but has not the single deep groove 

 which generally distinguishes the Cyclas cornea, and is more opake, 

 and blueish on the inside. 



cornea; Cyclas testd subrhombeo-globosd aquilaterali subdiaphand transversim 

 substriatd, sulco unico. 

 Shell somewhat rhombic and globular equilateral semitransparent, 

 with irregular transverse striae and a single groove. 



