232 PLEUROCERAS, AMALTHEUS. ETC. 



Family AMALTHEOID.E. 



PLEUROCERAS. Abdomen flat, with keel and channels well de- 

 fined ; keel crenulated ; channels vary from obsolete to deep and 

 well-defined, pilse swelling below, tuberculated ; genicular bend 

 prominent. Tubercles lateral, arranged along the line of envel- 

 opment. Umbilicus open. Ventral lobe narrow and but slightly 

 deeper than the lateral lobes ; the latter unequally divided. In- 

 ferior lateral lobe small, shallow, equally divided. Superior 

 lateral cell only partly exposed on the side, and together with 



the inferior lateral, unequally divided. 



Middle Lias. 

 PL. SPINATUS, Brug. T. 109, figs. 633, 634. 



AMALTHEUS. Abdomen acute, keeled and channeled ; whorls 

 compressed laterally. Keel crenulated, well defined. Tubercles, 

 when present, are in a single row along the line of envelopment. 

 Umbilicus open, with the sides of the whorls exposed or only 



partially covered. 



Middle Lias. 

 A. MARGARITATUS, d'Orb. T. 109, figs. 635, 636. 



Family CYCLOCERATID^E. This family is remarkable for 

 containing species which on the one side ally it with the 

 Liparoceratidne, and on the other with the higher Hildocera- 

 tidae. There is, however, a general agreement in the devel- 

 opment and in the septal characteristics, which unite them 

 in one family. The form is much more compressed laterally 

 than in the Liparoceratidre, and the tuberculations of the 

 pilre separate them from the Hildoceratidse. The young of 

 Tropidoceras Actaeon resemble the adults of Cyclocerax 

 Valdani, and the young of the last in their turn are like the 

 adults of Platypleuroceras latsecosta; thus all three genera 

 are closely connected by development. The abdominal lobe 

 is of about the same depth as the superior lateral ; the latter 

 is unequally divided into three minor lobes of variable 

 length, and there is only one auxiliary lobe exposed to view 

 on the side. Superior lateral cell is generally equally 

 divided, and of great breadth. Inferior lateral, narrower 

 and more prominent. 

 PLATYPLEUROCERAS. Abdomen nearly as broad, or broader 



than the dorsal side of the whorl. Pilae single, tuberculated, 



