260 OLCOSTEPHAM s. 



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 diameters of Peri- 

 sphincta?, are Per. Ch.a /'//'. I'ict., Enthymi, Pict., Malbo.</\ I'ict. 

 The most extivmc form of tin- whole series is Per. Leopoldianus, 

 which in the young state agrees closely with Per. radiatus, ~\m\ 

 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, MS yet, to resolve upon the generic sepa- 

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

 tion of the forms is so very small, and only becomes somewhat 

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

 ianus, d'Orb. 



It is remarkable that Perisphinctes Leopoldianus hears much 

 resemblance to Haploceras Beudanti in the form of the lobes ; 

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

 undoubtedly ,does not belong to the forms derived from Peri- 

 sphinctes. 



161 species. Jurassic, Cretaceous. 



PERISPHINCTES ARBUSTIGERUS, d'Orb. T. 112, figs. f>r>2. or,:*. 

 Jurassic. 



Genus OLCOSTEPHANUS, Ncum. 



The best known typical species of this genus, Olc. a^(irrit///u.<, 

 was placed In Perisphinctes b\ Waaircn. and. in fact. it. with its 

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

 should be separated from the i^enus Peri*i>/iin.rfr.. since it forms 

 a very well marked divergent series, and ditl'ers in several im- 

 portant characters from the typical representatives of the latter. 



The origin of the irronp of forms which we embrace under 

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

 from Peri*ji/u'm-fr. -rein- to have taken place far in the Kast. 

 and after completed dillerentiation of the type, migrated into 

 European districts. The intermediate form between ]'> ri- 

 *l>hin< !<.< and ()l<-<>*t<>i>li<i mis is represented by Olc. Cautleyi, Opp., 

 from the Indian .Jurassic, which shows the point of bifurcation 

 of the ribs pushed in to the umbilical an^le. Imi which in other 



