NO. 2.] DESCRIPTION OF THE FOSSILS. 91 



broader external side of the whorls, compare the diagrams of letter-press 

 figs. 14 and 16 b ; (3rd) in the less distinct sickle shape of the ribs, or its 

 total absence; (4th) in the less marked forward flexure of the ribs near the 

 external side; (5th) in the lobe-line. The second lateral saddle and the aux- 

 iliary saddles are scarcely narrowed at all in Cadoceras Nanseni. The full- 

 grown specimens of Cadoceras Nanseni differ from those of Cadoceras 

 Tchefkini principally, in that in the former species the sculpturing continues 

 distinct for a considerably longer time than in C. Tchefkini, and also in the 

 usually flatter form of Cadoceras Nanseni n. sp. 



The young whorls of Cadoceras Nanseni differ from C. sublceve Sow. 

 sp. in being less involute, with lower, externally broader whorls, with finer 

 and closer, less distinctly falciform ribs. 



As compared with the young whorls of Cadoceras modiolare d'Orb. sp., 

 those of Cadoceras Nanseni are lower and considerably more slender, with 

 considerably closer and finer ribs, and a wider umbilicus. 



The sculpture of the young whorls of Cadoceras Nanseni n. sp. resembles 

 somewhat that of Cadoceras Elatmce Nikitin 1 . The young whorls of 

 both species, however, differ widely in this particular, the young whorls 

 of Cadoceras Elatmce Nik. increase considerably in breadth at a very 

 early stage, when their diameter measures no more than about 25 mm.' 2 

 Moreover, the lobe-line in Cadoceras Elatmce is quite different from that in 

 Cadoceras Nanseni. The saddles are all considerably lower, the second 

 lateral lobe in Cadoceras Elatmcv Nik. is remarkably small, and the next 

 saddle very broad and low. Full-grown specimens of both species cannot be 

 mistaken for one another. 



The specimens of Cadoceras Nanseni n. sp. now before me are for the 

 most part embedded in clay-sandstone, partly in less sandy, marly rock. The 



1 S. Nikitin, 'Dcr Jura der Unigcgcnd von Elatma'. Lfg. 2, p. 14. From a large specimen 

 of Cadoceras Elatmce Nikitin, from Elatma, I prepared the inner whorls, so that I was 

 enabled to examine the Russian species at stages of growth corresponding to the figs. 

 2 and 3 on PI. II. 



' S. Nikitin. 1. c. fig 2, PI. VIII (X), fig. 47. 



