246 Mr. H. Seeley on Greensand Ammonites. 



young of A. auritus is the species A. splendens, and small shells 

 like A. Studeri become with age the species A. Raulinianus. 



Yet the other forms all have a value, though for convenience 

 they may be regarded as varieties of these types, which are but 

 subspecies of a larger group now named Ammonites permutatus. 



Ammonites (Crioceras) occultus. PI. X. fig. 1. 



Moderately compressed, flattened, with few whorls, which 

 rapidly enlarge, and are so closely coiled that, while the whorls 

 do not appear to have actually touched, the tubercles of the 

 back have impressed themselves into the underside of the suc- 

 ceeding whorl. The transverse outline of the last whorl is four- 

 sided. The back is flat, and the base is a little concave. The 

 sides round into the base, and approximate each other with in- 

 creasing rapidity as they near the back, into which they also 

 gradually round. The back is half as wide as the base, and one- 

 third the height of the side. In the earlier whorls the back 

 appears to have been more round. 



The shell is ornamented with a great number of moderately 

 elevated rounded ribs, which, below the middle of the side, are 

 slightly inflected forwards, as they are on nearing the back. At 

 the base of the side the ribs are collected in twos and threes, 

 forming elongated, elevated, obtuse tubercles ; they ascend the 

 side at about equal distances apart, and so pass over the back ; 

 but, at the angles which the sides make with the back, every 

 third or fourth rib developes a large elevated tubercle, the base 

 of which is at least as wide as the space between the ribs : the 

 tuberculated ribs are often stouter than the others. On the basal 

 side, where the ribs are bent forwards, are two impressed lines 

 marking the width of the back of the preceding whorl; the 

 space between the lines is rather more than a third of the width 

 of the base. 



The septa are indistinct. The dorsal lobe is twice as long as 

 wide, and extends over three ribs ; it has two large, bifurcating, 

 many-digited, terminal branches, and two branches on each side, 

 the lower one being large. The dorsal saddle is as wide as half 

 the height of the side, divided by one large and many smaller 

 branches. The superior lateral lobe is about as large as that on 

 the back, but longer; it terminates in a trifid branch, the cen- 

 tral ramus of which has three digits. The inferior lateral lobe 

 is short ; the basal lobe minute. 



This remarkable Crioceras was obtained by the Rev. Dr. Cook- 

 son from near Hunstanton. But I suspect that both it and 

 the Trigonia formerly named T. Hunstantonensis have been ob- 

 tained from the Drift. It has been liberally presented to the 

 University Museum. 



