University of Texas Bulletin 35 



The other forms found are mostly very similar to some from the 

 Sosio beds. Medlicottia Whitneyi is not only very nearly related to 

 M. Biirckhardti from our zone of Waagenoceras, but also to M.. 

 hifrons Gemm. from the Sosio beds. Gastrioceras altudense belongs 

 without doubt to the group of G. Zitteli. Paralecanites altudensis 

 cannot easily be compared with any other form described. 



We may add that Perrinites is much more highly developed than 

 anything found until now in the cephalopod-bearing sandstones of the 

 Artinsk; which indicates that this latter horizon is still older and that 

 our Perrinites beds represent perhaps a zone between the cephalopod- 

 bearing sandstones of the Artinsk and the Sosio beds. The fauna 

 of the Perrinites beds is more nearly related to that of the latter 

 horizon. 



We shall now try to show the relations between our lower horizons 

 (the zone of Prothalassoceras and zone of Uddenites) and the Artinsk 

 and Sosio beds. This comparison is somewhat hampered by the ex- 

 treme scarcity of ammonoids in the zone of Prothalassoceras and also 

 by the circumstance that a certain part of the zone of Uddenites has 

 been destroyed by erosion during the Permo-Carboniferous time. 



The only two ammonoids so far found in the zone of Prothalasso- 

 ceras, both of which show intimate relations to forms from the Artinsk 

 sandstone, are Prothalassoceras Welleri n. sp,. and Marathonites Har- 

 gisi n. sp. The only relatives of these forms seem to exist in the ceph- 

 alopod-bearing sandstone of the Artinsk. Prothalassoceras Welleri 

 with its characteristic simple suture, has a great similarity to Thalas- 

 soceras Gemmellaroi Karp., although it is certainly specifically dif- 

 ferent. The resemblance of the suture in both species is really sur- 

 prising, but as we shall explain in the description of our new species, 

 it is possible that Thaiassoceras Gemellaroi represents an immature 

 stage of a real Thaiassoceras and that larger specimens may show a 

 more complicate suture, in which case it would be proven that our 

 Prothalassoceras is a real precursor of Thaiassoceras. Just at present 

 we do not know the development of the suture of this latter genus. 



Marathonites Hargisi n. sp. has its nearest relative in Popanoceras 

 sp. indet. (cfr. Parkeri Heilpr. ) Karpinsky, but a complete comparison 

 cannot be made because in the Russian form the auxiliary saddles and 

 lobes are unknown. 



