576 me. l. p. spath osr [Dec. 1 9 13, 



• 



they were due to different age, in so far as the specimens from 

 Central European localities generally were of smaller dimensions 

 than those of Mediterranean origin. He stated that, with increase 

 in diameter, the umbilicus widened, and that the sides sloped 

 less steeply towards the ventral margin : that is, that in the adult 

 Peltoceras transversarium changed into P. toucasianum. 



After being separated again by A. de Riaz, 1 the two forms were 

 once more united by P. de Loriol, 2 who considered the occurrence 

 of a number of transitional forms as proof of their close affinity, 

 and who, moreover, regarded Quenstedt's type-figure as representing 

 an exceptional variety. Dr. H. Salfeld, 3 who in 1906 renewed 

 the discussion of the identity of the two forms, has been able to 

 demonstrate, however, the correctness of Quenstedt's original 

 drawing, 4 and has refigured the type of transversarius, together 

 with additional specimens. 



There can be no doubt that A. d'Orbigny's P. toucasianum can 

 clearly be distinguished from Quenstedt's form, even if we do not 

 accept Dr. Salfeld's interpretation of the former in its entirety. In 

 the true P. toucasianum the ribs are sigmoidal and strongly rursi- 

 radiate ; in P. transversarium they are simply curved backwards, 

 even in the young. According to Dr. Salfeld 5 also, the periphery 

 broadens in the adult P. toucasianum, whereas in P. transversarium 

 the narrowing of the periphery is a fairly constant feature, which 

 is the reverse of what Neurnayr had believed to be the case. 



If we consider the costation only, the change from one form into 

 the other also seems to take place the other way about : that is, 

 rather from P. toucasianum into transversarium than vice versa, 

 and we may, with Dr. Salfeld, therefore regard P. toucasianum as 

 the Mediterranean ancestor of the Birmensdorf form. 



A. d'Orbigny's ammonite, at a diameter of 75 millimetres, has 

 the following dimensions : — 



Umbilicus 39 per cent, of the diameter. 



Height of the last whorl ... 34 per cent, of the diameter. 



Gemmellaro's specimen, at 67 mm. diameter, shows : — 



Umbilicus 41 percent, of the diameter. 



Height of the last whorl 34 per cent, of the diameter. 



My slightly smaller specimen is almost equal in dimensions, and 

 for larger proportions the specimen of ' Peris-phinctes ' transversarius 



1 ' Description des Ammonites des Couches a Peltoceras transversarium de 

 Trept(l8ere)'1898, p. 54. 



2 ' Oxfordien Superieur & Moyen du Jura L^donien' Mem. Soc. Pal. Suisse, 

 vol. xxx (1903) p. 103. 



3 ' Beitrag z. Kenntnis des Peltoceras toucasi (d'Orb.) & P. transversarium 

 (Quenst.) ' N. Jahrb. vol. i (1906) p. 81. 



4 F. A. von Quenstedt, ' Cephalopoden ' 1849, pi. xv, fig. 12 & p. 199; and 

 ' Ammoniten d. Schwabischen Jura' 1888, pi. xci. fig. 26 & p. 830. 



6 Op. si/pra cit. p. 87, footnote. 



