NO. 2.] DESCRIPTION OF THE FOSSILS. 81 



1897. Ammonites (Gadoceras) Tchefkini d'Orb.? E. T. Newton and J. J. 



H. Teall, 1. c. Quart. Journ. Geol. Soc. London, vol. 

 53, p. 496, e. p. pi. XXXIX. fig. 5. (not fig. 4 and 6). 

 1897. Ammonites (Gadoceras) modiolaris Luid. E. T. Newton and J. J. 



H. Teall, 1. c. Quart. Journ. Geol. Soc. London, vol. 

 53, p. 497, e. p. pi. XXXIX. figs. 7, 8 (not figs. 9 and 10). 

 The species so widely spread in tlie middle Callovian of Russia is repre- 

 sented in the material collected by Prof. Nansen at Cape Flora, besides the 

 figured specimen by a number of impressions of young and by two fragments 

 of body chambers, most probably to be classed here. 



The well preserved specimen — PI. II. fig. 7 — broke so favourably, in 

 being prepared, that it permits of a considerably more accurate description 

 of the young whorls of this important species than has 

 hitherto been given by d'Orbigny and Nikitin. 



The young whorls — up to a diameter of 40 mm — are 

 moderately convex, and very involute. Four fifths of the 



_. ^ . height of each whorl is covered by the next whorl. On account 



Fig. 14. ° •' 



Gadoceras of the great involution, the umbilicus is very narrow and com- 

 sp. Diagram of paratively deep. (conf. the accompanymg diagram). 



PI. II. fig. 7; rpj^g specimen figured on PI. II. fig. 7 shows at a dia- 



nat. size. 



meter of 30 mm. the following dimensions: 



Diameter: 30 mm. = 1. 



Width of the umbiHcus 5-4 " = 0-18. 

 Height \ of the last 15 „ = 0-50. 

 Thickness / whorl 13 „ = 0-43. 

 These measurements correspond to the young specimen of Gadoceras 

 Tchefkini which Nikitin in 1881 figured ^ from the Jura of the Upper Volga 

 (Rybinsk, etc.). 



The section of the innermost whorls — up to a diameter of about 10 

 mm. - is thick, shortly oval. At a diameter of about 20-25 mm., the section 

 becomes higher. The greatest thickness of the whorl is then at about half 

 its height. Towards the exterior the transverse section of the whorl becomes 

 smaller. The outside is narrow but rounded 2. 



' 1. c pi. III. fig. 23. 



2 As in the illustration given by d'Orbigny: 1. c. pi. XXXV. fig. U. 



