GENEALOGY. 57 



RADICAL STOCK. 



Psiloceras calipht/llum, as supposed by Neumayr, may have been the radical 

 discoidal and smooth form from which Psil. planorbe originated. It is a very close 

 ally of this species, differing only in the sutures, and these, like those of other mem- 

 bers of the genus in the Northeastern Alps, are phylliform, and have, as we have 

 said, a triassic aspect. Although we are disposed to share Neumayr's opinion 

 that Psil. caliphylhim is the radical of all the Arietidas, we think, nevertheless, 

 that the evidence of the forms and sutures favors the theory that the Levis Stock 

 of the Central European province all sprang from Psil. planorbe. The sutures of 

 the normal Arietidae of Central Europe have less complicated margins than those 

 of Psiloceras and Caloceras of the same province; but these in turn are as a rule 

 less complex than those of the same genera in the Northeastern Alps. The 

 Arietidaa, therefore, can be characterized as having degenerated in respect to 

 the sutural margins of the septa. The degeneration of the sutures in Psiloceras 

 planorbe and in Cal. laqucum and Verm, spiratissimum enables one to see that this 

 tendency was general even in the Plicatus Stock; and it is probable 1 that the 

 Plicatus Stock, with the exception of Vermiceras, all originated in the North- 

 eastern Alps from Psil. calipliyllum. 



The degeneration of the sutures is due to an arrest of development followed 

 by the retention of nealogic characters, and is purely degenerative. This is, 

 however, accompanied by the evolution of a new character, an increase in the 

 depth and narrowness of the abdominal lobe, in the typical Arietidae. 



PLICATUS STOCK. 



Wsehneroceran Series. 



The interesting forms discovered in the Mediterranean province, and de- 

 scribed by Wahner in his " Unterer Lias," 2 show that the closest affiliation 

 existed between Psiloceras and the schlotheimian group. The genus Wsehne- 

 roceras, 3 described farther on, contains species like Wcehn. extracostatum, carvi- 

 ornatum, Panzneri, etc., which are transitional between Schlotheimia and the true 

 plicated species of Psiloceras. 



Schlotheimian Series. 



In this series 4 the number of forms having the pilae crossing the abdomen 

 with a peculiar forward bend, especially in Schlot. catenala, enable the observer 

 to see that a direct connection by transitional forms must have occurred between 

 this and Waehneroceras. The similarities are, however, not so close as to be 

 traceable in a series of connecting forms, and one is still left in doubt whether 

 the evolution of this series took place in an earlier formation than that of the 

 Planorbis bed, or in that bed itself. Suspecting that the nealogic stages of Schlot. 



i See Chapter IV. 8 Summ. PI. xi. fig. 7-10. 



2 Mojsis. et Neum., Beitr., II., 1882. 4 Summ. PI. si. fig. 3-6. 



