268 



THE LIAS AMMONITES. 



Genus — Arnicoceras, Hyatt. Fossil Cephalopoda, Bull. Mus. Camb., U.S.A., p. 73, 1868. 

 Ophioceras, Hyatt, ibid., p. 75 ; Discoceras, Hyatt, ibid., p. 76. 

 Coroniceras, Hyatt, ibid., p. 77 ; Asteroceras, Hyatt, ibid., p. 79. 

 Family — Discoceratid^e, Hyatt, ibid., p. 73. 

 Genus — Arietites, Waagen. Formenreihe des A. subradiatus, Beneckes geog. pal. Beitr., 



Band ii, p. 247, 1869, and Pabeont., Band xvii, p. 198, 18/0. 

 — Neumayr. Zeitscbrift Deut. geolog. Gesell., Band xxvii, p. 906, 1875. 



Shell discoidal, with a wide umbilicus. The sides of the whorls covered with straight 

 simple ribs, which often have tubercles developed on them near the ventral angle, as seen 

 in Arietites rofiformis, fig. 182, and still better shown in PI. IX, figs. 1,2. In all the 

 species the keel is large and prominent, and accompanied by two lateral channels (fig. 

 184). The mouth-border is simple, scooped away on the sides, and furnished with a 

 long, projecting ventral process. The body-chamber is from one to one whorl and a 

 quarter in length. 



Fig. 182. — Arietites rotiformis, 

 Sow. Side. 



Fig. 183. — Arietites ohtusus, FlG. 184. — Arietites 

 Sow. Side. ohtusus, Sow. Front. 



Fig. 185. — Section of Arietites 

 obtusus, Sow. 



The lobes are very characteristic (see Arietites bisulcatus, Bruguiere, Plate III, fig. 3). 

 The siphonal lobe is nearly as deep as it is broad, and the point of its adhesion to the 

 siphuncle is nearly in the middle of its depth. The principal lateral lobe does not reach 

 to half its depth, and is sometimes as broad as it is deep. The lateral saddle rises above 

 all the others and stands upon the ground of the principal lateral lobe, in general 

 double the height of the siphonal saddle. The lower lateral lobe is likewise much broader 

 than deep, and the columellar saddle is so small that it does not reach to half the height 

 or breadth of the lateral saddle and ends in two branches. 



The genus Arietites is readily distinguished from the other genera of the Aegocera- 

 tid^e. Its closest affinities are with Harpoceras and Amaltheus. It differs from 

 Harpoceras in the absence of bent, sickle-shaped ribs, the short body-chamber, the 

 different figuring of the lobe-line, and the want of a bivalved calcareous Aptychus. It 

 differs from Amaltheus in possessing a long body-chamber. In Amaltheus it is short, 

 from two-thirds to one-half of a whorl in length. The mouth-border is likewise simple, 

 the ventral portion ending in a long projecting process. The lobe-line is moreover 



