NO. 2.] DESCRIPTION OF THE FOSSILS. 



85 



The differences reported by Newton between his Ammonites Cadoceras 

 Tchefkini P d'Orb. and Cadoceras TchefUni of d'Orbigny and Nikitin are not 

 differences in distinguishing features of different species but differences in 

 age in different sized specimens of the same species. In full-grown specimens 

 of Cadoceras TchefUni, the number of ribs on the exterior side is a con- 

 siderably larger one than in smaller specimens, a larger number of secondary 

 ribs from the exterior surface being interspersed in the larger specimens than 

 on the whorls of younger stages. Newton is wrong in calling the ribs around 

 the umbilicus in Cadoceras Tchefkini "distinctly larger than those on the 

 back". In youth and on the shell they are not "larger" around the um- 

 bilicus, but higher and sharper, and towards the external side they be- 

 come lower and broader. Only in larger .specimens — of a diameter of 40 

 mm. and upwards — and on internal casts, when tripartite ribs appear, do 

 the ribs on the margin of the umbilicus become flatter and somewhat broader 



Among the specimens from Elmwood at Cape Flora, which Newton 

 figures as Amm. {Cadoceras) modiolaris Luid., figs. 7 & 8 on PI. XXXIX. 

 are undoubtedly to be referred to Cadoceras Tchefkini d'Orb. sp. 



If we complete the fragment in figs. 7, 8, which, according to the lobe- 

 line, corresponds almost exactly to half the width of a whorl, we get a cross- 

 section agreeing entirely with the transverse section of a whorl of a large 

 specimen of Cadoceras Tchefkini as drawn by Nikitin ^. 



Moreover the lobe-line of fig. 7 (especially as regards the development 

 of the second lateral saddle), agrees entirely with that of a specimen of 

 Cadoceras Tchefkini from Rybinsk before me. The lobe-line differs from 

 those, which are to be observed in Cad. modiolare Luid. of the English 

 authors (Cad. sublceve Sow. sp. and modiolare d'Orb. sp.), as the latter 

 species of a corresponding size shows a considerably more developed second 

 lateral saddle. 



While Newton maintains that the cast of the umbilicus of one of his speci- 

 mens just fits the umbilicus of "A. modiolaris" from the Kellaways Rock, we 

 maintain in the first place, that among the English Cadocerates Nikitin has also 



S. Nikitin: Rybinsk, etc. 1881, pi. III. fig. 21. 



