ORDER II AMMONOIDEA 585 
the sutures internally, and to multiply number of lobes and saddles in proportion, both 
externally and on the dorsum. First lateral saddles apt to divide in later stages into 
three distinct saddles, 
Family 22. Sphenodiscidae. Sutures slightly more complex than those of 
Engonoceratidaec, and with phylliform marginals; dorsals monophyllic. Young 
similar to those of Engonoceratidae 
in primitive species, and have con- 
cave venters and compressed volu- 
tions. Venter subacute or keeled 
in the adult, but later loses its keel, 
and becomes plano-convex or 
rounded. 
Sphenodiscus, Meek ; - Indoceras, 
Noetling; Lzbycoceras, gen. nov. 
(Fig. 1224). Type L. (Sphen.) 
Ismaéli, Zittel. Cretaceous. 
Family 23. Placenticera- 
tidae. Tuberculated and smooth 
forms, the young of which pass 
through a stage with smooth, flat, 
or concave venters similar to adult 
of Engonoceras. Venter remains 
flat or compressed until old age, 
when it becomes rounded. Sutures 
remarkable for their complex, 
irregular, ragged outlines, and 
narrowness of saddles. Their outlines in the young very similar to those of Spheno- 
discidae. 
Placenticeras, Meek; Diplacomoceras, gen. nov. Type D. (Amm.) bidorsatus, 
Schliit. sp. Cretaceous. 
Family 24. Engonoceratidae. Compressed shells with numerous saddles and 
concave or flat venter. Costae nodose in some species, but usually fold-like and 
sometimes merely broad, low lateral swellings. Sutures simpler than in Spheno- 
discidae, but of the same type. 
Engonoceras, Neum.; Knemiceras, Bohm; Neolobites, Fischer; Protengonoceras, 
gen. nov. Type P. (Amm.) Gabbi, Hyatt = Amm. pedernalis, Gabb. Cretaceous. 

Fic. 1224. 
Libycoceras Ismaéli, Zittel sp. Upper Senonian; Libyan 
Desert west of Oasis Dachsel. 
IX. ACANTHOCERATIDA. 
A series of more or less uncoiled genera connected through the transitional Acantho- 
ceratidae with normal Cretaceous shells having costae interrupted on the venter at some 
stage or throughout life, although in most forms continuous during later stages of the 
ontogeny. 
Family 25. Acanthoceratidae. Young slender, discoidal, without tubercles, 
and section rounded until late in neanic stage. Sutures have deeply-cut outlines, 
and owing to frequent presence of only six lobes and saddles simulate those of 
Leptocampyli, but are not of the same type. First lateral saddles broad and 
bifid; ventral lobe straight, deep, and has a truncated siphonal saddle. External 
characters similar to some forms of Hoplitidae. Some species have costae bifur- 
cated, more or less interrupted, and with only the ventro-lateral row of. tubercles 
or none. 
Acanthoceras, Neum.; Thurmannia, gen. nov. Type 7. (Amm.) Thurmanni, Pict. 
