240 Mr. H. Seeley on Ammoniies 



enlarges, the ribs strengthen, the umbilical tubercles, as well as 

 those bordering the back, are more elevated ; and thus A. 

 splendens varies into a new form, which it may be useful to dis- 

 tinguish as A. cratus. 



Ammonites cratus. PI. XI. fig. 2. 



Form inflated, with half-embracing whorls, and a mouth nar- 

 rower than high, though wider than the umbilicus. The sides 

 are convex. The fiat back is less than half as wide as the 

 mouth. 



Around the umbilicus is a row of twelve large and elevated 

 spines, separated by spaces wider than their bases : they send 

 thickenings down the umbilicus, into which the most convex 

 part of the side rounds, abutting on the embraced whorl. They 

 also give rise to extremely elevated narrow ribs, separated by 

 wider, deep, concave channels, curving moderately mouth ward. 

 Three ribs are collected in each spine, and there is a free one 

 between each two bundles. About one half reach the back single, 

 and terminate each in a strong, elevated, tubercular thickening, 

 which extends obliquely forward into the middle of the back; 

 the remainder unite in twos at the dorsal angle, and form similar 

 tubercles. These tubercles are so arranged as to give a slightly 

 dendritical aspect to the back. 



Septa symmetrical, consisting of the dorsal lobe and, on each 

 side, three (? or more) lobes. The dorsal lobe is rather longer 

 than wide. The dorsal saddle, much wider than the lobe, is 

 centrally cleft. The superior lateral saddle is as wide as the 

 dorsal, has a single notch on each side, and terminates in three 

 large trifurcate branches. The other parts of the suture have 

 the same structure as those described, but get rapidly smaller. 

 As the forms depart from the original series, and the whorls 

 get more inflated, the septa become less and less unsymmetrical. 



This extreme form is not common. The largest example in 

 the University Museum measures 3| inches high, and has septa 

 to the end. 



Another branch of the series now passes on rapidly to A. au- 

 ?itus > Sow., with which should be united A. Guersanti, Pict. 

 (not D'Orb.); for our specimens are almost identical with fig. 7, 

 pi. 5 of the ' Gres Vert/ differing only in having rather fewer 

 tubercles on the back — a character which is the only one to 

 show that the figure is not copied from D'Orbigny's A. auritus, 

 pi. 07. vol. i. 'Terr. Cret.' 



Ammonites leptus. PI. X. fig. 5. 

 Few-whorled, greatly compressed; sides nearly flat; back 



