116 GREY CHALK MARL. 



crested with a row of small tubercles, from which the radii arise. The 

 latter proceed obliquely over one-fourth of the volutions, where they form 

 another set of tubercles, from whence they diverge into two branches, 

 each terminating in a tubercle on the outer margin. The carene is 

 smooth, and forms a prominent acute keel, having a row of opposite tu- 

 bercles placed on each margin. 



In a suite of fifty specimens, in which every individual presented some 

 peculiarity, three principal varieties were observable, each passing in- 

 sensibly into the other. 



Va?\ subplana. Tab. xxi. fig. £. 



The volutions depressed, radii linear, inner row of tubercles obscure, 

 external margin crenated, keel but shghtly elevated, aperture sagittate. 



Some specimens of this variety are nearly smooth, and the keel so 

 much compressed, that without the aid of numerous examples, their re- 

 lation to the tubercular variety could not have been ascertained. 



Var. intermedia. Tab. xxi. figs. 7, 8. 



In this variety the volutions are rather depressed, the radii broad and 

 well defined, the tubercles small and distinct, the external margin tu- 

 berculated, the keel prominent, and the aperture sagittate. 



I'his is the usual form of the species, and holds an intermediate rank, 

 between the smooth and tubercular varieties. 



Var. tuhcrculata. Min. Conch. Tab. clxxvi. figs. 1, 2, 3, 4. 6. 



Volutions subrotund ; radiations short, thick, nodular ; tubercles elon- 

 gated, very prominent ; keel acute ; aperture roundish. 



A very beautiful variety, distinguished by its projecting tubercles, of 

 which Mr. Sowerby's figure 1, affords an excellent example. The inner 

 rows of tubercles are almost effaced, but the marginal and intermediate 

 sets are strongly reheved, and in some examples become spinous. From 

 the thickness of the volutions, the aperture is obovate. 



LocaHties ; very abundant at Hamsey, Middleham, Stoneham, &c. 



SI. Ammonites ductus. 



Dicoidal, subumbilicate, volutions depressed, half inserted, transversely 

 jadiated; radii aniudar, distant, bifurcate, undulated; umbilicus expanded, 



