OF PLANORBIS AT STEINHEIM. 75 



mediate form and characteristics. Fig. 13, line «, pi. 1 must also be joined to this 

 sub-variety. The form has not a very close resemblance owing to the deflection of the 

 mouth, but the third carina is well developed and the shell is intermediate to figs. 14 and 

 16 same line. 



Second sub-variety. 



It is not possible to arrange these varieties in a line with reference to Steinheimensis, 

 and therefore the successive numbering of the sub-varieties means nothing so far as 

 the genesis and relative rank is concerned. The forms figured on pi. 1, line d, figs. 1-4, 

 have both the first and second carinae, and the sulcation on the upper side is indicated. 

 Fig. 16, line e, shows a passage form from these directly to Steinheimensis. The under 

 sides of the whorls in this variety are rounder than in the first variety, stouter and not so 

 involute. 



Other sub-varieties might be described in the transition forms, such as fig. 1, line &, 

 pi. 1. This is an extremely thin shell, more or less flattened on both the upper and 

 lower sides of the whorls, and with the faintest possible sulcation. 



Near the mouth or the upper side, it is evidently very closely allied to such flattened 

 forms of Steinheimensis as fig. 9, line m, pi. 1, or intermediate between these and the 

 still more flattened form on line n, fig. 8, which leads into forms figs. 9-12 on the same 

 line, belonging to the true tenuis group. We must also add to this list fig. 6, line n, pi. 

 1, which fills the gap between fig. 7 of the tenuis group and Steinheimensis. 



The specimens described in the Lower Steinheun Period, as occurring in the Cloister 

 Kidge rocks show with considerable clearness that PI. tenuis is really a derivative of PI. 

 levis, and that the transition forms here described between this and PI. Steinheimensis 

 must be accounted for either as hybrids or as descendants from Cloister Ridge species. 



Variety Kraussii. 

 Planorbis Kraussii Klein, Jahresh. Wiirt., 1847, plate 1, fig. 28. 



PI. multiformis Kraussii HUgend., Monast. K. Preu. Akad. d. Wissensch., Berlin, 

 July 1866, pi., fig. 12. 



PI. Kraussii Sand., Conchyl. d. Vorwelt, Supp., p. 646. 



This species at first sight appears to be separable from PI. Steinheimensis on the one 

 side and PI. tenuis on the other. The close and thorough examination of the shells, 

 however, gradually obliterates all distinctions. It can, therefore, if one chooses, be 

 properly considered one of the varieties of PI. steSiLnsis- 



If we compare the full grown shell with the young of PI. tenuis, line/, fig. 1-7, pi. 2, 

 we find that the young of certain forms are with difficulty separated from PI. Kraussii, 

 e. g., fig. 4, line /, and fig. 3, line d, pi. 2. The young of the more immature of the 

 tenuis forms are like the specimens of PI. Kraussii. 



This would answer very well for all the forms with flattened upper sides, figs. 1-8, line 

 d, and figs. 13-16, same line, pi. 2, in which a triquetrus-like outline is attained by the 

 flattening of both the upper and lower sides. This explanation, however, hardly applies 

 to such forms as figs. 9-12, and 17, on the same line. These are almost purely Stein- 

 heimensis-like, so much like the latter that if they had been found in the same formation, 

 I should have called them by that name. 



