from the Cambridge Greensand. 245 



and, compared with the umbilical spines, small. The umbilicus 

 is deep, and about as high as or higher than the whorl opposite 

 the mouth. 



Ammonites Studeri, Pictet & Campiche. 

 A. Studeri, P. & C. ; Ste. Croix, pi. 30. 



Few-whorled, more than two-thirds-embracing, greatly in- 

 flated, with a flattened back, and umbilicus as high as the whorl 

 opposite the mouth, and bordered by a row of large spines. 



The mouth is rather wider than high, and nearly twice as 

 wide as the back. The exposed part of the ventrum is inflated 

 and nearly horizontal ; but quite on its border, where the side 

 rounds into it, is placed the row of ten massive tubercles; these 

 are much-elevated cones, with bases a fourth the height of the 

 side ; above them the sides are flat, and converge to the back. 

 From each spine arise two or three ribs, which are slightly 

 curved mouthward, obtuse, and not much elevated till reaching 

 the margin of the back, where they terminate each in a thick- 

 ening which can scarcely be termed a tubercle, and which extends 

 a short way on to the back. Those of the two sides are alter- 

 nate, so that the slightly convex back presents a distant approach 

 to the kind of zigzag ornament which marks the back of A. 

 Raulinianus. 



In the most typical specimens the septa are almost effaced ; 

 they appear nearly, if not quite, symmetrical. The dorsal saddle 

 is wide and unequally cleft by a small branch. The superior 

 lateral lobe is long, with two small notches on each side, and 

 terminates in three trifid branches. The inferior lobe is small, 

 and in the line of the tubercles. 



Specimens identical with those figured in the ' Paleontologie 

 Suisse^ are rare; but small specimens having all the characters 

 of ornament the same, and difi'ering only in being relatively 

 much less inflated, are by no means imcommon. But it is with 

 doubt that I have cast these in with A. Studeri; for the young 

 state of A. Raulinianus is identical. 



The adult shells of this type vary much in the degree of ele- 

 vation and number of the ribs, as well as the way in which they 

 are gathered in the tubercles. 



I believe the facts given in this paper compel the union under 

 one specific type of every shell it describes after Ammonites coelo- 

 notus. Throughout the series the variations in the septa are 

 insignificant. Every variety of shape is nothing but an inflated 

 form of Ammonites splendens modified by the different develop- 

 ment of crenuljE and ribs. The four chief species. Ammonites 

 splendens, A. awitus, A. Raulinianus, and A. Studeri, are in- 

 separably linked together by intermediate forms; while the 



