GREY CHALK MARL. 117 



sides smooth, with a marginal row of oblique tubercles ; ambit convex, em- 

 braced by the radii ; aperture ovato-sagittate. 



The volutions, although compressed, have a slight degree of convexity, 

 and are ornamented by transverse radiations, that arise from a row of 

 small tubercles on the inner margin. Each radius divides into two branches, 

 which pass with a gentle sweep across the ambit, and unite with the cor- 

 responding undulations of the opposite side ; small obhque tubercles are 

 placed on each radius at the point of bifurcation. The volutions are three 

 in number, the inner ones being three-fourths concealed. The dorsum, 

 or ambit, is gently undulated by the radii. 



But one specimen has been discovered, the dimensions of which are 

 as follows : longest diameter 3-8 inches ; width of the outer volution, two- 

 fifths of the diameter: transverse diameter of the aperture 1-1 inch. 



It is scarcely necessary to remark, that although this species approaches 

 to A. varians, in having bifurcating radiations, and a row of tubercles on 

 the inner margin, yet it is widely separated from it by the rounded form 

 of the back, and other obvious differences. 



Locality. Middleham. 



32. Ammonites yo/ccf^M*. Tab. xxi. fig. 6. 12. 



Depressed, subumbihcate, volutions deeply inserted, transversely 

 radiated; radii plicated, falciform, extending down the sides of the 

 umbilicus ; umbilicus small, with crenulated edges ; dorsum flat, narrow, 

 with a longitudinal sulcus ; margin pUcated ; aperture sagittate, siphun- 

 culus ? 



This rare and elegant species is almost flat, the longest diameter ex- 

 ceeding its greatest thickness nearly four-fifths. The volutions are slightly 

 enlarged in the centre, but are contracted at the ambit into a narrow flat 

 keel, with a sulcus down the middle, and dehcate plicated edges. The 

 radii are very slender at their origin in the umbilicus, but gradually in- 

 crease in breadth, and passing obliquely to the centre of the volutions, 

 make a sudden curve towards the margin, where they terminate in 

 obtuse folds. The form of the septa, and the situation of the siphunculus, 

 are unknown. 



