34 FOSSIL MOLLUSCA OF THE CHALK. 



specimens having it distinctly trifid, as it is figured in the 'Pal. Fran?.,' fig. 106, while 

 others have it unequally bifid. 



A marked variety is found in the Chalk with siliceous grains of Somersetshire and 

 Dorsetshire, in which the ribs are nearly obsolete ; but the general form of the shell, the 

 tubercles, and the outline of the septa identify the species. 



M. d'Orbigny unites A. hippocastanum, Sow., to A. Rhotoma<j/ensis, in which I cannot 

 coincide ; the unequal ribs of the former contrasting too strongly with the extreme regularity 

 of the latter to admit of their belonging to one species. 



20. Ammonites Sussexiensis, MantelL Plate XV, fig. \. 



Ammonites Sussexiensis, MantelL Geol. Sussex, t. xx, fig. 2. (Not t. xxi, fig. 10.) 



A. testa discoided, costatd, tuberculatd : anfractihus tumidis, quadrato-rotundatis •■ costis 

 rectis, later aliter simplicibus, bituberculatis ; junioribus, dor so bi- vel tri-divisis, tri-tuber- 

 culatis ; adultis, dor so continuis simplicibus : umbilico magno -. septorum lobis lateralibus, 

 primo bijido, secundo trlfido. 



Shell discoidal ; whorls tumid, flattened at the back and sides, with nearly a square 

 section, very little concealed ; ornamented with sixteen or eighteen tuberculated ribs ; on 

 the sides of the whorls the ribs are all equal, straight, rounded, and bearing two large 

 tubercles, one near the umbilicus, the other near the back ; but the dorsal portion of the 

 ribs varies with the age of the shell ; at three inches diameter (the youngest examined) 

 each rib divides on the back into two or three ribs, which reunite at the opposite tubercles 

 and bear three small tubercles each ; a little later it is only the middle tubercle which is 

 divided into two ; after this the rib is continued across the back as in one form of A. 

 Mhotomagensis : umbilicus very large, concealing hardly one fifth of the inner whorls. 

 The superior lateral lobe of the septa is bifid, the inferior lateral lobe trifid. 



Diameter, 6 inches ; width, 2 inches ; height of aperture, 2 j inches. 



Found occasionally in the Grey Chalk near Lewes, Sussex. 



The two shells originally figui-ed in the Geology of Sussex by Dr. Mantell, under the 

 name of A. Sussexiensis, belonged to two difierent species, to one of which the name of 

 A. Rhotomagensis had just before been given in France ; the other being the species above 

 described as A. Sussexieiisis. The distinctions between them having been overlooked, they 

 were united together by Sowerby in the ' Mineral Con chology,' vol. vi, p. 25, and all 

 subsequent authors having adopted the same view, A. Sussexiensis has been viewed as a 

 synonym of A. Rhotomagensis : the adult shells, though a good deal alike, may be 

 distinguished by the number of their ribs, but in their younger stage the differences in the 

 dorsal ribs and tubercles are too great to allow of any confusion between them. 



