260 OLCOSTEPHANUS. 



allied, which, differing little amongst themselves, become more 

 and more distinct in their sculpture from the ground form, ap- 

 proaching the group Per. radiatus ; these intermediate forms, 

 whose inner whorls still present the true characters of Peri- 

 sphinctae, are Per. Chaperi., Pict., Enthymi, Pict., Malbosi., Pict. 

 The most extreme form of the whole series is Per. Leopoldianus, 

 which in the j^oung state agrees closely with Per. radiatus, but 

 becomes quite smooth in old age ; hand in hand with the change 

 in the sculpture, a modification in the lobular markings also 

 steps in. 



I have not been able, as 3'et, to resolve upon the generic sepa- 

 ration of these forms from PerispMnctes, since the range of varia- 

 tion of the forms is so very small, and onl}- becomes somewhat 

 greater in two forms, viz : in Per. radiatus, Brug., and Leopold- 

 ianus, d'Orb. 



It is remarkable that Perisjyhinctes Leopoldianus bears much 

 resemblance to JIaploceras Beudanti in the form of the lobes ; 

 but according to the form of the inner whorls, Ifapl. Beudanti 

 undoubtedly does not belong to the forms derived from Peri- 

 sphinctes. 



161 species. Jurassic, Cretaceous. 

 Perisphinctes arbustigebus, d'Orb. T. 112, figs. 662, 663. 

 Jurassic. 



Genus OLCOSTEPHANUS, Neum. 



The best known typical species of this genus, Olc. astierianus, 

 was placed in Perisphinctes by Waagen, and, in fact, it, with its 

 numerous relatives, belongs to this stem ; I believe, however, it 

 shovild be separated from the genus Perisphinctes, since it forms 

 a very well marked divergent series, and differs in several im- 

 portant characters from the typical representatives of the latter. 



The origin of the group of forms which we embrace under 

 Olcostephanus, is not to be sought in Europe, but the divergence 

 from Perisp)hinctes seems to have taken place far in the East, 

 and after completed differentiation of the type, migrated into 

 European districts. The intermediate form between Peri- 

 sphinctes and Olcostephanus is represented b}^ Olc. Gautleyi, 0pp., 

 from the Indian Jurassic, which shows the point of bifurcation 

 of the ribs pushed in to the umbilical angle, but which in other 



