from the Cambridge Greensand. 243 



than the back, with two notches on each side, and terminating 

 in two small branches. The dorsal saddle is considerably wider. 

 The upper lateral lobe is long and narrow, terminating in three 

 small branches, and having one branch on each side. The lower 

 lobe is very small, and in a Hue with the umbilical tubercles. 



It is unwillingly that this form has been described separately 

 from A. aaritus, of which it is a badly marked variety, differing 

 chiefly in a different inflation of the shell. It has been classed 

 by the Swiss naturalists with A. Raulinianus ; but in this series 

 names nowhere mark real boundaries or breaks. 



The young shell so nearly resembles A. Studeri that I am 

 unable to discover any character not common to the two forms. 

 This circumstance in no way invalidates the conclusions arrived 

 at on the affinities of this shell ; for all the forms, from A. Fit- 

 toni to the most extreme variation of A. Studeri, have round 

 backs in the young state ; and if this shell is more inflated than 

 usual, that is because the adult is one of the most gibbous of the 

 series. 



There are many variations from this form, in one of which 

 the ribs disappear; in another the dorsal tubercles gradually 

 become obsolete, while the back gets narrower and the sides 

 more convex, till at last the back as a flat region becomes obso- 

 lete, and the alternate ribs almost meet in alternate thickened 

 terminations along its middle. The ribs curve much toward the 

 mouth as they near the back. This small shell with a dendrous 

 back is probably immature. 



Ammonites Salteri, Sharpe, Cret. Moll. pi. 23. figs. 5 & 3, is 

 a variation including those more compressed forms in which the 

 dorsal spines are reduced to tubercles and the umbilical spines 

 are small and the ribs slightly elevated. 



In a variety which may be named A. tetragonus the shell has 

 flat sides and a flat back, and the mouth nearly quadrate. On 

 the lower third of the side is a row of ten or eleven small spines, 

 which are moderately elevated and separated by wide intervals. 

 At each dorsal angle is a row of about eighteen tubercular spines, 

 larger than the umbilical row, and, though short, directed late- 

 rally, which widens the back. These spines are connected by 

 ribs, which on the last half-whorl are very slightly elevated, and 

 ultimately become obliterated. Three appear to have always 

 diverged from each umbilical spinous eminence. The last half- 

 whorl is devoid of septa. Diameter 2 inches ; width of mouth 

 f inch. 



Another shell (PL XI. fig. 6) of the same group scarcely differs 

 as a variety from A. Renauxianus, P. & C. Ter. Cret. Ste. Croix, 

 pi. 31. figs. 2-5 ; D'Orb. T. C. vol. i. pi. 27. Shell compressed, 

 with very few whorls, one-third-embracing, and enlarging very 



17* 



