ARIETITES SCIPIONIANUS. 289 



prominent ; sulci deep ; area above the angle covered with oblique folds ; ribs nearly 

 straight for two-thirds their length, from suture to angle, where each rises into a small 

 prominent tubercle ; they then turn sharply towards the aperture, stretch obliquely 

 forward on the area, vanishing at the outside of the sulcus. Whorls, five or six, inflated 

 at the sides ; inner whorls one-fourth concealed, outer whorl one-third the diameter of 

 the shell ; aperture subquadrate ; diameter one inch and a half. 



Dimensions. — Diameter of the figured shell, 42 millimetres ; height of the last whorl, 

 15 millimetres; thickness of ditto, 11 millimetres; amount of involution, 4 millimetres. 



Description. — After carefully examining this fossil I have arrived at the conclusion 

 that it is not a distinct form of Arietites, but a variety of Amaltheus Haivskerensis, 

 Young and Bird = spinatus, Brug., which has acquired an increased number of ribs, and 

 converted the grooves on each side of the keel into sulci, resembling those of a true 

 Arietites. Simpson says, in some specimens the radii are still closer, and the charac- 

 teristic tubercles become nearly obsolete, until it can scarcely be distinguished from 

 A. geomeiricus, Phil., which is in fact Am. spinatus with a notched keel. The rock in 

 which A. subnodosus is fossilised appears to be the Marlstone of the Yorkshire coast, 

 this species will, therefore, be noticed more in detail in the description of Amaltheus 

 spinatus, Brug. 



Arietites Scipionianus, d'Orbigny. PI. XIII, figs. 1 — 3; PI XIX, figs. 8—10. 



Ammonites Scipionianus, oVOrbigny. Pal. Francaise, Terrain Jurassique, p. 207, 



pi. li, figs. 7, 8, 1842. 

 — ■ personatus, Simpson (Bean's MSS.). Monogr. on Ammonites, p. 9, 

 No. 6, 1843. 



— — Simpson. Fossils of the Yorkshire Lias, p. 38, 1855. 



— Scipionianus, Quenstedt. Der Jura, p. 69, pi. viii, fig. 1, 1858. 



— — Dumortier. Depots Jurassiques du Bassin du Rhone, 



torn, ii, p. 33, pi. viii, fig. 12 ; pi. ix, fig. 1, 1867. 



— — Collenot. Descrip. Geologique de l'Auxois, p. 235, 1873. 

 Arietites — Tate Sf Blake. Yorkshire Lias, p. 287, pi. v, fig. 3, 1876. 



Diagnosis. — Shell discoidal, compressed, acutely carinated ; ribs numerous, slightly 

 bent between the suture and marginal angle, where they terminate in blunt tubercles 

 in young and middle-aged shells ; sides flat, with from twenty-four to thirty-eight ribs. 

 Siphonal area narrow, rather angular, with smooth sloping declivities, and a narrow, acute, 

 prominent keel. Aperture hexagonal, compressed, and sharply angulated. 



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