AMMONITES— ZOOLOGICAL, ETC. 



143 



15th Group, The Capricorni, Buch. — Shell with very- 

 convex whorls, ornamented v/ith prominent, simple, straight 

 ribs, without tubercules or spines. Back wide, often pre- 

 senting a wider surface than the sides. Septa composed of 

 lobes, formed of unequal parts,* and of saddles, formed of 

 equal parts. Dorsal lobe the longest ; the lobes of the sides 

 wide. All the species are from the Jurassic formation {A, 

 Capricornus, angulatus, &c.) 



SPECIES VS^lTH A ROUND CONVEX BACK. 



16th Group. The Heterophylli, d'Orb. — Shell com- 

 pressed, whorls almost always embracing, rarely visible in the 

 umbilicus. The sides are smooth, slightly striated or grooved. 

 Back small, very convex. Septa symmetrical, divided into a 

 great number of highly foliated lobes, formed of unequal 

 parts, and saddles, generally formed of equal parts. Dorsal 

 lobe almost always shorter than the superior lateral one. 

 The numerous branches of the lobes have the saddles be- 

 tween them, ornamented at the superior part with large 

 foliations or rounded masses, more or less divided, and pecu- 

 liar in appearance. They may be placed in two divisions : — 

 1st, the species having the saddles formed of unequal parts, 

 comprising A. HeterophyUus of the Jurassic rocks; 2nd, the 

 species with the saddles divided into equal parts, all peculiar 

 to the lower Cretaceous rocks {A incertits, infundibulum, se- 

 mistriatus, tortisulcatus, Calypso, Gnettandi, semisulcatus, 

 Tethys^ Morelianus, picturatus, Terverii, diphillus, Rouyanus, 

 of the lower Neocomien, and A. Velledae and alpinus, from 

 the Gault or Lower Green Sand). We thus perceive that 

 the species from the Cretaceous rocks are clearly distin- 



* 1 have not seen the lobes of this group ; the description is taken from 

 the figure given by Von Buch. It is the only group which I have not per- 

 sonally examined. 



