204 UPPER, OR FLINTY CHALK. 



in length ; each spine has a groove on the under, and a corresponding 

 ridge on the upper surface. They generally project from the shell, but 

 in some instances He close on the surface. 



The beaks are convex, and incurved ; the ears small, and even ; the 

 margin neatly denticulated. A specimen cleared from the chalk, exhibited 

 no muscular impression. 



There are several varieties of tliis species, of which the following are 

 the most remarkable : 



Var. a. With both valves gibbous, and but few spines. 



b. Valves depressed, spines numerous. 



c. Valves gibbous, ribs regularly convex and even. 



d. , ribs channelled near the front. 



This shell is one of the most common productions of the Upper chalk, 

 but is less frequent in the lower beds ; the hardness of the Sussex chalk 

 renders it exceedingly difficult to clear the specimens, without destroying 

 the spines. 



Localities. The chalk-quarries in every part of the South Downs. 

 SiHceous casts of the interior of the shell, occur among the flints on 

 Plumpton Plain. 



69. Plagiostoma Brightoniensis. Tab. xxv. fig. 15. 



Obovate, depressed, longitudinally costated: posterior side eared; an- 

 terior side lunulate, concave, small, acuminated ; margin crenulated. 



The length of this species is 3 inches; greatest width 2-5 inches ; thick- 

 ness of the united valves, 1 inch: the convexity of the valves is nearly equal. 

 The costae are somewhat flattened, and channelled towards the anterior 

 margin ; there are about thirty on each valve. The anterior depression 

 is elongated, and marked with longitudinal striae ; the posterior side is 

 short, and eared. The Hues of growth are very distinct. 



Locality. The specimen figured is from the Upper chalk of Brighton. 



70. Plagiostoma Hoperi. Tab. xxvi. figs. 2, 3, 15. 



Transversely ovate, obhque; valves convex, covered with numerous 

 diverging striae ; posterior slope short ; anterior slope concave, elongated, 

 obliqviely striated; ears unequal; margin entire. 



