58 PSEUDOCERATITES OF THE CRETACEOUS. 
LiByCocERAS ISMAELE (Zittel). 
Sphenodiscus ismaelis Zittel, Handb. der Paleeontologie, Vol. II, p. 451, fig. 631. 
Libycoceras ismaeli Hyatt, 1900, Zittel’s Text-book of Palaeontology, Vol. I, p. 585. 
According to Zittel this has three principal lobes and saddles. 
Locality: Libyan Desert. 
Age: Upper Senonian. 
SPHENODISCUS Meek. 
This genus is apparently a close ally of Engonoceras, judging by its 
sutures and external characters, but the development shows it to be very 
distinct. In two species, plewrisepta and lenticularis, the young were 
examined, and in these there were no indications of a stage having a flat or 
concave venter. In the nepionic stage the species is rounded on the venter 
which in the neanic changes directly to a form with an acute venter and 
flattened side like that of the adult. ‘There are three principal lateral 
saddles derived from the division of the primitive first lateral saddle. 
The earliest stage described in S. lenticularis exhibits sutures like those 
of the adult of Neolobites, but they are also very similar to those of some 
species of Hngonoceras having longer, narrower, and more phylliform saddles 
than usual, like Hngonoceras subjectum. There is, however, in Engonoceras 
and in Neolobites also a very distinct persistent first lateral. These peculiar- 
ities in development of the sutures, as well as the acute venter, heighten the 
resemblances to the shell figured by Zittel as S. ismaelis. This is generically 
distinct and here described as Libycoceras, but Sphenodiscus might be a 
modified form of the same series in which the keel had become incorporated 
with the venter and with more complex sutures, whereas its development 
does not indicate descent from any of the genera haying flat and tuberculated 
venters. The external characters of this genus are often so uniform that 
the sutures as arule are the only means of distinguishing the species. The 
exceptions to this rule are to be found in the American forms S. pleurisepta, 
and perhaps acutum, and the European representative of the former, S. binck- 
horsti, which have lines of tubercles and faint coste. The keel is solid, and 
the siphuncle being placed well within the sutures of the siphonal saddles, 
is protected by a ridge of the stony filling of the camere and is not, there- 
fore, easily laid bare when the shell is removed from casts. 
