AMMONITES. 39 



rise into a tubercle near the umbilicus, and also on each side of the back : the old shell is 

 nearly smooth, with faintly marked obsolete ribs. 



Diameter, 3 inches ; height of the last whorl, 1£ inch ; width of aperture, 1 inch. 



From the Grey Chalk of Hamscy, near Lewes. The only specimen seen is in the 

 British Museum, from the collection of Dr. Mantell. In France it has been found in the 

 Grey Chalk near Saumur, and in the Upper Green Sand of Mondragon, near Orange. 

 (I)' Orbigny, loc. cit., p. 319.) 



With some general resemblance to the variety sub-plana of A. varians, this species is 

 distinguished from that and all the other known species of the Chalk by the depression of 

 the sides of the whorl near the umbilicus, and the peculiar sigmoidal flexure of the ribs, 

 in which characters it is related to A. serpentinus, Schloth., and several other species of 

 the Lias and Lower Oolite. 



25. Ammonites Velled^e, Michelm. Plate XVII, fig. 7 ; and Plate XIX, -fig. 6. 



Ammonites Velled/e, Michelin. Magasin de Zoologie, t. xxxv. 



— — D' Orbigny. Paleont. Fran?. Terr. Cret., vol. i, t. lxxxii. 



— — Pictet and Roux. Foss. des Gres Vert des env. de Geneve, t. ii, fig. 1. 



A. testa discoided, compressd, transversim subtilissime costatd : anfractibus paucis } 

 lateribus complanatis , dorso rotundatis : costis tenuibus, rectiusculis, numerosissimis, anfrac- 

 tum totum amplectentibus : umbilico minimo : aperturd elongato-ovatd, septorum lobis late- 

 ralibus trifidis. 



Shell discoidal compressed, with few whorls, flattened on the sides, and rounded at 

 the back ; crossed by numerous fine, equal, thread-like ribs, which embrace the whole 

 whorl : umbilicus very small :* aperture elliptical : lateral lobes of the septa trifid. 



Diameter, If inch; height of the last whorl, 1 inch; width of aperture, ^ inch. 



Found rarely in the Upper Chalk near Norwich, and in the Grey Chalk of Ventnor, 

 Isle of Wight. In France and Savoy it is found in beds of the age of the Gault, attaining 

 a diameter of 7 inches. 



26. Ammonites naviculars, Mantell. Plate XVIII, figs. 1 — 3, 5, 8. 



Ammonites navicularis, Mantell. Fossils of the South Downs, t. xxii, fig. 5. 



— — Sowerby. Min. Conch., 555, fig. 2. 



— — D'Orbiffny. Prodrome, vol. ii, p. 146. 



— Gentoni, Brongniart. Env. de Paris, t. vi, fig. C. 



— Mantelli, jy Orbigny. Paleont. Fran?. Terr. Cret., t. 103. 



— Milletianus, Dixon. Geol. of Sussex, t. xxix, fig. 15. 



* Owing to the specimen, pi. xvii, fig. /, consisting of very soft Chalk, the umbilicus has been too 

 much excavated, and is represented too large. 



