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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. (?) (Mm. Conch., PL 125). 



Nautilus intermedius, D'Orb. (Terr. Jur ass., 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. intermedius 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. 



