50 FOSSIL MOLLUSCA OF THE CHALK. 



each side, and a hollow canal in the middle : ribs falcate, very variable in number and 

 size, and, rising from tubercles on the edge of the umbilicus, they extend forward to the 

 middle of the side of the whorl, then suddenly turn backward, and again curve forward 

 to the dorsal tubercles ; sometimes they are five in number, broad and depressed ; in 

 other specimens they are small and numerous on the inner half, and broad and few on 

 the outer half of the whorl ; frequently they are well marked on young shells, and 

 gradually disappear with age ; and in many specimens they are altogether wanting : three 

 rows of tubercles on each side of the whorl, one at the edge of the umbilicus, and two 

 near the edge of the back ; the umbilical tubercles vary both in size and number ; when 

 there are no ribs, the tubercles are few in number, and very large ; but they diminish in 

 size, and increase in number, in proportion to the development of the ribs ; the dorsal 

 tubercles are usually large, but they vary in number from 15 to 30, being fewest and 

 largest where there are no ribs : umbilicus small and deep, allowing about one third of the 

 inner whorls to be seen : aperture rhomboidal : septa with three trifid lateral lobes on 

 each side. 



Diameter, 1| inch ; height of last whorl, f inch ; width of aperture, J to f inch. 



Found abundantly in the Chloritic Marl of the Isle of Wight ; the Chalk with 

 silicious grains at Chardstock, Somersetshire ; and the junction bed of the Chalk and 

 Upper Green Sand, near Warminster. 



In the first part of this Memoir, I followed M. D'Orbigny and most other modern 

 palaeontologists in uniting together the Ammonites falcatus and curvatus of Mantell and 

 Sowerby ; but, as better materials have appeared, I have been obliged to separate them 

 again. Both are most variable species, and there are varieties which can only be distin- 

 guished with difficulty. The only invariable distinction which I have found between them 

 is in the termination of the superior lateral lobe, which is trifid in A. curvatus and bifid in 

 A. falcatus ; see PI. XXIII, figs. 1 c and 2. In comparing the external forms, it will be 

 found that the more tuberculated varieties aU belong to A. curvatus, the flatter, ribbed 

 varieties to A. falcatus; but there are intermediate forms combining falcate ribs with 

 dorsal tubercles, which can hardly be distinguished without the aid of the lateral lobe. 



41. Ammonites Salteri, Sharpe. Plate XXIII, figs. 3 and 5. 



A. testa discoided, cost aid, tuber culatd : costis ter-natis flexuosis, bi-tuberculatis, ad 

 dorsum, interruptis : dorso utrinque tuberculato, tuberculis alternantibus : umbilico parvo : 

 aperturd oblongd : septorum lobo dorsali longissimo. 



Shell discoidal, with few w^horls, ornamented with ribs and tubercles on their sides : 

 back plain, and slightly elevated in the middle, with a row of tubercles on each side ; these 



