from the Cambridge Greensand. 231 



width or depth of the internal casts of those in A. Mayorianus, 

 and did not bend into the umbilicus, but terminated at the base 

 of the side. The ribs on the back are more elevated and wider 

 apart; the number of whorls fewer. The sulci are arranged 

 relatively to each other like those figured in 'Cret. Mol.^ pi. 12. 

 fig. 3. It will be readily seen, on referring to that plate, how 

 much better the above description coincides with the Liewes 

 Chalk fossil than with the ancestral race figured beside it. 

 Hence Sowerby's name, specially used, should be restricted to 

 the Chalk form to which it was originally given ; while D'Or- 

 bigny^s would with more propriety be preserved for the Conti- 

 nental fossil, with which ours corresponds. 



In the variations from A. Mayorianus, the whorls gradually 

 become more compressed, with flatter sides rounding into the 

 umbilicus, which is relatively smaller. There are from five to 

 eight sulci. These specimens frequently occur with part of a 

 whorl devoid of septa at a diameter of an inch and a half. In 

 the young state (diam. | inch) the sides are flat, and a little 

 converging to the back ; and as the sulcations are scarcely im- 

 pressed, the shell has the aspect of A. Beudantii. 



Some specimens, which died small, exactly correspond with 

 Mr. D. Sharpens figure of A. octosulcatus in the number of sul- 

 cations, their relative size, and the way they pass over the back : 

 though the form of the whorl is not quite the same, yet so near 

 are the two, that I have no doubt of the propriety of regarding 

 A. octosulcatus as a slight variety of A. Mayorianus. 



There are no specimens at Cambridge of A. Griffithsii-, but, 

 judging from Mr. Sharpens figure and description, it cannot be 

 regarded as other than a variation from A. Mayorianus. 



The type of shell here discussed is one of the more abundant 

 of the Greensand Cephalopods, occurring in this neighbourhood 

 wherever that deposit is worked. 



Ammonites Weistii, Sharpe, 'Chalk MoUnsca/ pi. 21. fig. 3. 



Few-whorled, inflated; back round; mouth semilunar, much 

 wider than high; umbilicus moderately open, with its deep 

 border forming a large angle with the side of the shell. 



Ornamented with about twenty-two wide, straight ribs, nearly 

 all of equal length; every third rib is somewhat thickened on 

 the sides as it neai's the umbilicus, while the ribs between these 

 thickened ones frequently become obliterated before reaching 

 the edge of the umbilicus. 



Septa simple, consisting on each side of three lobes. Dorsal 

 lobe marked on each side with two simple digits. The dorsal 

 saddle^ half as wide again as the dorsal lobe, is indicated by a 



