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GENEALOGY. Sy 
RADICAL STOCK. 
Psiloceras caliphyllum, as supposed by Neumayr, may have been the radical 
discoidal and smooth form from which Ps7. 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. caliphyllum is the radical of all the Arietidse, 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 Psi/. planorbe. The sutures of 
the normal Arietidxe 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 
Arietidx, therefore, can be characterized as having degenerated in respect to 
the sutural margins of the septa. The degeneration of the sutures in Ps¢loceras 
planorbe and in Cal. laqueum and Verm. spiratissmum enables one to see that this 
tendency was general even in the Plicatus Stock ; and it is probable’ that the 
Plicatus Stock, with the exception of Vermiceras, all originated in the North- 
eastern Alps from Psil. caliphyllum. 
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 Arietide. 
PLICATUS STOCK. 
Weehneroceran Series. 
The interesting forms discovered in the Mediterranean province, and de- 
scribed by Wiihner in his “Unterer Lias,’? show that the closest affiliation 
existed between Psiloceras and the schlotheimian group. The genus Weehne- 
‘roceras,’ described farther on, contains species like Wehn. extracostatum, curvi- 
ornatum, Panznert, ete., which are transitional between Schlotheimia and the true 
plicated species of Psiloceras. 
Schlotheimian Series. 
In this series* the number of forms having the pile crossing the abdomen 
with a peculiar forward bend, especially in Schiot. catenata, enable the observer 
to see that a direct connection by transitional forms must have occurred between 
this and Wehneroceras. 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 Schiot. 
1 See Chapter IV. $ Samm. Pl. xi. fig. 7-10. 
2 Mojsis. et Neum., Beitr., II., 1882. 4 Summ. PI. xi. fig. 3-6. 
