590 
Cenoceras. 
This genus was described in my Genera of Fossil Cephalopods to 
include a number of the Nautili of the Trias and Jura which should 
be separated. I propose now to limit the genus to those forms 
which, like Cenoceras intermedium, as figured by D’Orbigny, have 
trigonal ananepionic substages with subquadragonal metanepionic 
volutions and a dorsal furrow in the nepionic stage. The ephebic 
stage is also more or less quadragonal, with the dorso-ventral longer 
than the transverse diameters and the lateral zones convergent. 
The umbilical shoulders are prominent and the umbilical zones 
broad and at right angles to the plane of the coil. The sutures 
have ventral and dorsal saddles only in the first and second septa. 
The annular lobe and dorsal lobe are apt to develop very early, in 
some species certainly in the third septum. ‘The siphuncle is near 
the centre in the first septum and subsequently varies from dorsad 
to ventrad of centre, but is never near either the venter or the dor- 
sum. ‘The ornamentation has both longitudinal ridges and trans- 
verse bands, but the former may or may not be present in adults. 
CENOCERAS INTERMEDIUM. 
NAUTILUS INTERMEDIUS, Sow. (?) (A@in. Conch., Pl. 125). 
NAUTILUS INTERMEDIUS, D’Orb. (Zerr. Jurass., Pl. 27). 
Loc., Balingen, Middle Lias. 
Pl: xi, Figs. 15-18. 
I feel considerable doubt whether Figs. 17 and 18 are really the 
young of C. zvfermedius and the name is taken solely on D’ Orbigny’s 
authority. It has, however, a peculiar compressed form and obvi- 
ously a large umbilical perforation in the ananepionic and a sub- 
quadrate outline in the metanepionic, with siphuncle ventrocentren 
as in most specimens of this species. ‘The ana- and metanepionic 
substages have no dorsal furrow. 
Figs. 15 and 16 show the paranepionic and ananeanic substage of 
a specimen with the siphuncle dorsocentren, evidently an unusual 
position, since several other specimens of nearly the same age have 
it ventrocentren and in adults it is still nearer the venter. The 
whorl is tetragonal in the paranepionic, with well-marked ventral, 
lateral and dorsal lobes in the sutures and annular lobes. The dor- 
sal furrow is also well developed and the umbilical perforation, 
restored with a dotted line in Fig. 15, must have been quite large. 
