248 CONCHYLW—DITHYRA. 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 

 diverofent teeth in the other valve: the lateral teeth are trans- 

 verse, flat and somewhat pointed, placed on each side the hinge, 

 single in one of the valves, and closing between double ones iu 

 the other valve. 



^- Siihorbiculares, aqnilaterales. 1^3, 

 somewhat orbicular and equilateral. 



rivicola. CvcLAs testd suborhiculari (Pquilaterali convexd opacd, strils 

 iransversis confertis regnlaribus. 

 Shell somewhat orbicular equilateral convex and opake, with 

 regular close-set transverse striai. 



Tab. nost. II, fig. 1.3. 

 3Itis. 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 tlie inside. 



cornea. Cyclas testd subrhombeo-globosd fequilaterali snbdiaphand 

 transversim substriatd, snlco unico. 

 Shell somewhat rhombic and globular equilateral semitrar « 

 rent, with irregular transverse striae and a single oroove. 



