282 PLEUROCERAS. AMALTHEl'S. ETC. 



Family AMALTHEOID.E. 



Pleuroceras. Alxlomen flat, with keel and channels well de- 

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

 well-defined, pila^ swelling helow, tubercnlated ; genicular bend 

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

 oinnent. Umbilicus open. Ventral lobe narrow and but slightly 

 deeper than the lateral lobes ; the latter unequall}^ 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 oul}' 

 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 

 Liparoceratidiv, and on the other with the higher Hildocera- 

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

 opment and in the septal characteristics, which imite them 

 in one family. The form is much more compressed laterally 

 than in the Liparoceratida?, and the tuberculations of the 

 pilse separate them from the Hildoceratidae. The yoimg of 

 Tropidoceras Actaeon resemble the adults of Gycloceraa 

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

 adults of Platypleuroceras latsecosta ; thus all three genera 

 are closely counected by development. The abdominal lobe 

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

 is u:nequally 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. Pilse single, tuberculated, 



