AMMONITES. 37 



22. Ammonites Cenomanensis, D'Archiac. Plate XVII, fig. I. 



Ammonites Cenomaneksis, TfArchiac. Mem. de la Soc. Ge'ol. de France, 2me serie, 



vol. ii, pp. 62 and 78 ; Histoire des Prog, de la Geol., 

 vol. iv, p. 347, note. 



— — IfOrhigny. Prodrome, vol. ii, p. 146. 



— WooLLGAEi, Id. Pal. Fran?. Terr. Cre't., vol. i, t. 108, figs. 1—3. 



— ViELBAXCii ? Id. Prodrome, vol. ii, p. 189. 



A. testa inflatd, transversim costato-tuberculatd ; anfractibus quadratis ; costis nume- 

 rosis, rectis, oetate juniori ad dorsum quinque-tuberculatis, adultd simplicibus, continuis, 

 hitubercidatis ; dorso latissimo, utroque latere tuberculato. 



Shell inflated ; whorls nearly square, crossed by about sixteen thick, straight, equal 

 ribs ; each rib starts from a large tubercle on the margin of the umbilicus, and bears 

 another larger tubercle at the side of the back ; the very young shell has not been seen • 

 but at 4 inches diameter there are three tubercles on the dorsal part of each rib, between 

 the two larger ones already mentioned; at the diameter of S or 9 inches these three 

 tubercles are lost in a strong straight rib crossing the back, and forming right angles at 

 its continuation with the lateral ribs ; in the still older shell, of a foot in diameter, the 

 ribs are less prominent, but the two tubercles at the sides of the back project 1| inch. 



Largest diameter, 1 foot ; width, 5 inches ; height of aperture, 4 inches. 



Common in the Grey Chalk of Kent, Surrey, and Sussex. 



This Ammonite is so closely related to A. Rhotoma^ensis, that it may perhaps be only 

 a variety of that species. The principal differences are in the ribs of the old shells, which 

 in A. BJiotomac/ensis become less and less distinct, till they finally disappear ; in A. Ceno- 

 manensis grow very prominent and square, and finally run into large projecting horns. 

 It appears to have been distinguished by M. D'Archiac and M. D'Orbiguy indepen- 

 dently. The latter author first confounded it with A. Woollgari, of Mantell, with which 

 it has little in common ; but M. D'Archiac gave it the name which it must retain. 



23. Ammonites hippocastanum, Sowerby. Plate XVII, figs. 2, 3, and 4. 



Ammomtes hippocastanum, Sowerby. Mineral Concliology, t. 114, fig. 2. 



A. testa injlatd, costatd, spinoso-tuberculatd ; anfractibus paucis, celeriier cresce?itibus, 

 angulatis ; costis 12 — 20 valde incequalibus, tuber culatis ; tuberculis 7 seriatis, inaqualibus ; 

 lateralibus interdum spinosis •• dorso lato, 5 tuberculato : umbilico parvo, prof undo : aper- 

 turd transversim rhomboided. 



Shell gibbose, with few broad and angular whorls, which increase rapidly in size, 

 ornamented with transverse, unequal ribs, and seven rows of large unequal tubercles : 



7 



