22 FOSSIL MOLLUSCA OF THE 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 y^yths of an inch. 



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

 grains, throughout the Southern Counties ; and also in the Green Sand of Blackdow^n. It 

 occurs iu beds of the same age in France. 



A.falcatus is related to J. lautus 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. 



H 



4. Ammonites variaxs, Sow. Plate VHI, figs. 5 — 10. 



Ammoxites varian's, Sowerby. Min. Conch., t. 1/6, lower figures only. 



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



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



— — UOrbigny. Pale'ont. Franc. Terr. Cret., t. xcii, figs. 3, 4, & .5. 



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



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



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



A. testa discoided, radiatd, tuberculatd, carinatd ; radiis numerosis jlexuods, plerum- 

 qi/p furcatis, bi-vel tri-tubercidatis ; tuberculis parvis ; dorso carinato, lateribm angulatis, 

 tuberctdatis, carina lavi ; timbilico mediocri ; aperturd tnincato-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 wdth 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 w'ith 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 aiTanged 

 under the four following varieties : — 



1st Var. siib-fuberculata, 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-wav between the fork of the ribs and the back : makinoj four rows of 



