AMMONITTDiE. 73 



Amalthese. 

 Shell generally flattened and carinated, the last whorl of the 

 spire covering a large part of the preceding ; sutural line with 

 several auxiliary lobes. Aptychus simple, corneous, only known 

 in the Jurassic forms. 



Amaltheus, Montfort, 1808. 



Distr. — 68 species. Triassic, Jurassic and Cretaceous. A. 

 ma7'garitatus, d'Orb. (xxxviii, 26, 27). 



Periphery sharpened or carinate ; ribs when present, absent 

 at this part or broken up into tubercles or folds ; the geologi- 

 call}^ older forms with spiral strife on the external layer of the 

 shell, which cori-esponds to the wrinkled layer of the Arcestas. 

 Body-chamber short, one-half to two-thirds of a whorl long ; 

 margin of aperture simply emarginate, with long, external pro- 

 cesses, ending in spoon-shaped extremities, sometimes bent out- 

 wards or inwards. Lobes usually strongly incised, siphonal 

 lobe shorter than the first lateral, lobular bodies broadly wedge- 

 shape. Umbilicus open, with the sides of the whorls exposed 

 or only partially covered. 



PLEUROCERAS, Hyatt, 1868. (Prionotropis, Meek, 1876.) Per- 

 ipher}^ flat, with keel and channels well defined ; keel crenulated ; 

 channels vary from obsolete to deep and well-defined, pilse 

 swelling below, tuberculatecl ; genicular bend prominent. Tuber- 

 cles lateral, arranged along the line of envelopment. Umbilicus 

 open. Ventral lobe narrow and but slightly deeper than the 

 lateral lobes ; the latter unequall}^ divided. Inferior lateral lobe 

 small, shallow, equally divided. Superior lateral cell only partly 

 exposed on the side, and together with the inferior lateral, 

 unequally divided. Scarcely distinct from Amaltheus. Middle 

 Lias, Cretaceous. A. spinatus^ Brug. (xxxviii, 24, 25). A. (Prio- 

 notropis) M^ooZgrar^, Mantell (xxxvii, 10, 11). 



OXYNOTICERAS, Hyatt, 1874. Periphery carinated in the young 

 shell, rounded in the adult. A. Guibalianus, d'Orb. (xl,50, 51). 

 Lower Lias. 



PTYCHiTES, Mojs., 1875. Shell covered with undulated radiating 

 plications ; exterior lobe shallow ; exterior saddle but little 

 elevated ; first lateral saddle very high ; saddles dentate or 

 slightly branched. This group, which corresponds to the Plicosi 

 of Beyrich and the Rugiferi of Oppel, may be considered an 

 ancestral form of Amaltheus. Six Triassic species. A. Studeri^ 

 Hauer. 



ScHLOENBACHiA, Neumayr, 1875. 



Dedicated to the geologist, Schloenbach. 



Syn. — Mortoniceras, Meek, 1876. 



Distr. — 46 species. Cretaceous ; Europe and United States. 



