205 
there is neither a keel or a channel in the siphonal area. The 
body-chamber is short, and the structure of the Aptychus is 
unknown. The number of the lobes is variable. In addition 
to the siphonal lobe and the principal and smaller laterals 
there are from two to four auxiliaries present. The ramifi- 
cations of the principal laterals are not symmetrically divided, 
and in the forms from the Neocomian strata the lobes have 
a more simple structure than those found in the Upper 
Cretaceous beds. This genus has a very wide stratigraphical 
range from the Lower to the Uppermost stages of the 
Jurassic series, and likewise in the Neocomian and upper 
Cretaceous formations. 
Genus SrepHanoceras, Waag.—The general form of the shell ” 
is very variable in this genus; the ventral side is in general 
large and round, without keel, border, or channels. The sculp- 
ture is often highly ornamental. From the suture rises a short, 
stout rib, which often ends in a tubercle at the middle of the 
side. From the tubercles two or three smaller ribs arise, which 
pass over the ventral surface, and unite with their fellows 
on the opposite side. Sometimes the tubercles are wanting, 
and a number of fine encircling ribs make up the entire 
sculpture. 
The mouth-border is often provided with broad lateral lappets, 
which are produced and inflected, as in Stephanoceras Bracken- 
ridgii, fig. 62, where they are very conspicuous, and often 
well preserved in this species. 
Fria. 59, Fie. 60. 
Stephanoceras Blagdeni. Sow. 
