206 UPPER, OR FLINTY CHALK. 



of its margin, give it a peculiar neatness *." The specimen figured and 

 described by Mr. Sowerby, was found in the chalk near Lewes. I have not 

 been so fortunate as to discover another example of the united valves. 



Tab.xxvi. fig. 21, represents the inner surface of a mutilated specimen 

 of the lower valve. 



Locality. Upper chalk, near Lewes. 



72. Dianchora obliqua. Tab. xxv. fig. 1. Tab. xxvi. fig. 12. 

 Obliquely obovate; upper valve convex, marked with numerous di- 

 verging striae ; margin serrated. 



The free valve is convex, and marked with upwards of 50 prominent 

 striee, separated from each other by fine sidci, and terminating in an 

 acutely serrated margin. The beak is acuminated ; the ears small, and un- 

 equal. The lower valve, by which it is attached to other bodies, has 

 its inferior surface finely granulated; its sides are erect, wide at the 

 anterior and posterior slopes, but very narrow in front. The posterior 

 side is more elevated than the anterior, and the upper valve is in conse- 

 quence inclined obhquely. 



This species is distinguished from D. lata by its ovate form, and 

 numerous striee ; and from D. striata, (the only other known species) by its 

 obliquity, striated surface, and denticulated margin. 



Tab. xxv. fig. 1. is a perfect specimen ; it shews the triangular cavity 

 beneath the beak, formed by the union of the valves ; the smooth, erect 

 border of the lower, and the striated surface of the upper valve. 



Tab. xxvi. fig. 12. The upper, or convex valve, detached; collected by 

 Col. Birch. 



Localities. Upper chalk, near Lewes, and Brighton. 



73. Ostrea . Smith's strata, Upper chalk. Figs. 5, 6. 



The shells of the genus ostrea are so irregular in form, and the same 

 species, in different stages of its growth, exhibits such dissimilar appear- 

 ances, that it is very difficult to arrange them with precision. This con- 

 sideration has induced me to relinquish the attempt to distinguish those 

 * Min. Conch. Vol. i. p. 184 



