UPPER, OR FLINTY CHALK. 205 



This shell is nearly related to P. rigida, and P. ovalis of Sowerby, 

 (Min. Conch. Vol. ii. tab. 114) from which it is distinguished by the in- 

 equahty of the ears, and the deep elongated lunette of the anterior side. 

 The general form of the shell is also somewhat dissimilar, and the strige 

 are less distinct than in the fossils above mentioned. 



This species presents considerable variety both in its form, and mark- 

 ings, but the differences observable, are not sufficiently constant to require 

 notice. It is more or less obliquely oval, but forms a larger segment of a 

 circle, than is strictly imphed by that term. The length and width nearly 

 correspond ; and the thickness of the united valves equals about half the 

 length. The striae, or rather sulci, are undulated, and distinct, in some 

 examples, but in others are altogether wanting. The lunette, or depres- 

 sion on the anterior side, is very deep, and extends from the beaks to 

 the commencement of the front ; it is marked by numerous oblique stria^, 

 which are decussated by the lines of growth. The ears are plicated ; that 

 on the anterior side is small, and obscure, but the other is distinct. The 

 beaks are small, and terminal. 



I have named this elegant shell in honour of the Kev. Henry Hoper, 

 A.M. of Portslade, to whose kindness I am more indebted, than his 

 modesty will permit me to acknowledge. 



Tab. xxvi. fig. 2. A specimen of the depressed variety with but few 

 striae. 



fig. 3. A shell in which the strige are distinct. 



— fig. 15. The united valves, the upper one rather displaced; 



this example exhibits the lunette on the anterior 

 side. 



Locahties. Upper, and Lower chalk, near Lewes. 



71. Dianchora lata. Tab. xxvi. fig. 21. 



Semicircular ; beak prominent ; free valve plain. 



The obhquity of this species is scarcely observable. The " lines of 

 growth being slightly marked, ard the gentle convexity of its form, added 

 to the indistinctness of the few striae upon its surface, and the sharpness 



