238 Mr. II. Sceley on Ammonites 



I have seen (perhaps twenty-five) show a wide amount of varia- 

 tion. The fossil figured (PI. X. fig. 2) is one of the most com- 

 pressed forms; and from it the umbilicus gets higher, the whorls 

 thicker, the ribs more numerous and less elevated, till at last, 

 to judge from fragments, the section of a whorl must have 

 been wider than high. In the form described the dorsal channel 

 is a third the width of the back, but in the widest form it is 

 only a ninth. 



A variety occurs in which the whorls are nearly half-embracing 

 (PI. X. fig. 3), flattened on the sides, rounding on the back, and 

 step-like around the umbilicus, ornamented with about thirty- 

 two rather elevated wide ribs separated by sulcations of about 

 equal width. The ribs are generally alternately long and short, 

 and terminate in fifteen umbilical tubercles. Aperture rather 

 higher than wide. Diameter 1^ inch minimum. 



MM. Pictet and Campiche, in their work on the fossils of Ste. 

 Croix, pi. 27. f. 2, have referred this type to A. falcatus of Mantell. 

 But at Cambridge no specimen of A. falcatus has ever occurred, 

 nor do the ribs vary in the least so as to approach that fossil 

 more than is seen in the specimen figured. The roundness of 

 the back and every feature of the ribs are matter for distinction ; 

 hence, and especially as the distribution is different, the forms 

 are separated. It is, no doubt, nearly related to A. falcatus, 

 having a channelled back and ribbed sides. 



Diam. 2j inches; septa to the end. 



Ammonites splendens, Sow. 



A. s/)Zewr7ws,Sow.M.C.t.l03; Pictet, Gres Vert, pi. 6. fig. 6; D'Orb.pUtf. 



A. Yittoni, D'Arch. 



A. auritus, D'Orb. T. C. vol. i. pi. 65. figs. 3 & 4. 



Shell compressed, with a small umbilicus, high, flattened 

 sides, and a very narrow flat back. The umbilicus, about as 

 high as the mouth is wide, and never more than a third the 

 height of the whorl, is shallow, with the horizontal ventrum, 

 which rounds into the side, not much deeper than the unem- 

 braced part of the whorl on which it abuts. The sides of the 

 whorls are very slightly inflated, and converge, so that the back- 

 is only half as wide as the base. The mouth is less than half 

 the height of the shell. The sides of the cast are smooth, 

 or marked only with a few broad flexuous ribs scarcely elevated. 

 The dorsal angles are each crenated, with a row of minute tu- 

 bercles, which send slight thickenings a short way down the 

 sides. 



Commonly in larger specimens the lower half of the side is 

 slightly inflated, so that the upper half looks more compressed : 

 the same peculiarity occurs rarely in specimens from Folkestone. 



