22 FOSSIL MOLLUSCA OF THF CHALK. 



growth ; but fig. 8 is a very remarkable variety, in which the tubercles have almost taken 

 the place of the ribs, of which only a few are to be seen in a very young stage. 



Diameter 2 inches ; thickness -poths of an inch. 



Found sparingly in the Grey Chalk, the Chloritic Marl, and the Chalk wdth siliceous 

 grains, throughout the Southern Counties ; and also in the Green Sand of Blackdown. Tt 

 occurs in beds of the same age in France. 



A.falcatus is related to J. lautm and A. tuherculatus, which have a similar canal down 

 the back ; but the peculiarly curved ribs readily distinguish it from those species, while 

 its dorsal canal distinguishes it from several species which have ribs with a similar 

 double curve. 



4. Ammonites varians. Sow. Plate VIII, figs. 5 — 10. 



Ammonites vakians, Sowerby. Min. Conch., t. 176, lower figures only. 



— — Brongniart. Envir. de Paris, t. vi, fig. 5. 



— — Bronn. Lethaea Geogn., t. xxxiii, fig. 2. 



— — jy Orhigny. Paleont. Fran?. Terr. Cret., t. xcii, figs. 3, 4, & 5. 



— — var. INTERMEDIA and var. sub-plana, Mantell. Fossils of the 



South Downs, t. xxi, figs. 2, 5, and 7. 



— Brongniartii, Haan. Aram, et Goniat., No. 43. 



A. testa discoided, radiatd, tuberculatd, carinatd ; radiis numerosis jlexuosis, pleruni- 

 que furcatis, bi-vel tri-tuberculatis ; tuberculis parvis ; dorso carinato, lateribus angulatis, 

 tuberculatis, carina laevi ; wnbilico mediocri ; aperturd truncato-sagittatd. 



Shell discoidal, ornamented on the sides with numerous flexuous, simple, or forked 

 ribs, each of which bears a small tubercle at its rise on the edge of the umbilicus, and a 

 larger tubercle at its termination on the side of the back, and in some varieties an 

 additional tubercle at the point of bifurcation ; umbilicus rather small and well defined ; 

 whorls half concealed ; back narrow with a prominent keel, and a row of tubercles at each 

 side ; section of the whorls considerably higher than broad, sagittate with a truncated 

 top ; aperture with the upper part of the whorl projecting forward, and produced into a 

 blunt point. Septa with 5 or 6 trifid, subdivided lateral lobes. 



Ammonites varians^ as its name imphes, assumes many forms, which may be arranged 

 imder the four following varieties : — 



1st Var. sub -tuber culata, PI. VIII, figs. 5, 6, and 8. The young shell has about 

 fifteen slight tubercles round the umbilicus, connected by very faint ribs, with about 

 thirty well-marked tubercles at the edge of the back : the adult shell has the ribs more 

 prominent and less numerous, about twelve rising from the umbilicus, and increasing to 

 about twenty at the edge of the back, an additional distinct tubercle occurs at the 

 bifurcation of each rib, and a row of fainter tubercles usually occurs on the branches of the 

 divided ribs, half-way between the fork of the ribs and the back ; making four rows of 



