CYCLOCERA8. TROPIDOCERAS. 233 



and extending across the rounded abdomen, as in Planiceras. 

 The septa are minutely divided by minor lobes, very closely set. 

 The abdominal lobe is deep ; sides abrupt. Superior lateral veiy 

 HJMTOW. deeper than the abdominal, and profusely branching. 

 Inferior lateral not as deep as superior lateral, and of about the 

 same breadth and general aspect. Abdominal cell large and 

 serrated. Superior lateral very broad, about the same height as 

 the inferior lateral. 



Middle Lias. 

 P. LATECOSTATA, Sowb. T. 108, fig. 62C>. 



CYCLOCERAS. AbdonKMi rounded or keeled, not so broad us 

 the dorsal side of the whorl. Pihv single, tuberculated, and not 

 extending across the abdomen in the keeled species. Youiii> 

 smooth for the first two or three whorls, then become ribbed. 

 Keel appears at an earlier stage of growth than the pila\ Septa 

 not so minutely divided by minor lobes, and the larger lobes less 

 dentritic than in Plalypleuroceras. The abdominal lobe of 

 medium depth and quite broad. Superior lateral of medium 

 breadth and considerable depth. Inferior lateral about two- 

 thirds as broad and dec]) as superior lateral. One small auxiliary 

 lobe exposed laterally. Superior lateral cell broad and depressed. 

 Inferior lateral more prominent and nnrrower; small auxiliary 

 cell exposed on the side. 



Middle Lias. 



C. VALDANT, d'Orb. T. 110, figs. <>42. Ol:{. 



TROPIDOOKKAS. A IK! omen invariably keeled, much narrower 

 than the dorsal side of the whorl. Pihx> single, smooth or tuber- 

 culated in the same species, do not extend across the abdomen in 

 any species. Young are smooth for one or two wdiorls. Keel 

 and pila? appear simultaneously. Septa have a more complicated 

 aspect than in the preceding genus, the minor lobes being deeper 

 and more numerous. The abdominal very broad at the bottom, 

 narrower above. Superior lateral lobe narrow, and about the 

 same depth as the abdominal. Inferior nearly the same, but less 

 branching than the superior lateral One auxiliary lobe exposed 

 on the side. Abdominal cell very broad. Superior and inferior 

 lateral cells very irregularly divided by minor lobes. One small 

 auxiliary lobe exposed on the side. 



Middle Lias. 



TROPIDOCERAS AOT/KON, <VOrb. T. 10S, figs. 627, 628. 

 30 



