1888 .] 
23 
LHyatt. 
generate and then resemble those of the adult of Psil. planorbe. 
The sutures of Psil. planorbe var. levis are obviously intermediate 
between those of Psil. Caliphyllum and Caloceras. The sutures of 
the latter are also intermediate between those of Psiloceras and 
Vermiceras, so that a most perfect series exists in Central Europe 
between the more complicated outlines common in Psiloceras and 
Caloceras and those of Vermiceras, which are simpler and purely 
Arietian in outline. The sutures have been, therefore, to a certain 
extent, arrested in their development and evolution in the normal 
forms of the Arietidse and this is a marked characteristic entirely 
compatible with progression in other characters. 
In studying the genetic history of each series by itself, species 
after species and variety after variety, it has been found practicable 
to attempt to follow out the migration of forms on the same horizon 
from one fauna to another, and, though no doubt very imperfectly, 
to make a beginning in the direction of tracing the evolution of 
faunas chorologically. The results of this work are very sug- 
gestive, and it may be said with approximate accuracy, that some 
faunas are autochthonous or give rise to new forms and, that oth- 
ers, which I have called residual faunas, are not favorable for the 
evolution of new species or varieties. 
It seems very probable that the basin of the Northeastern Alps 
was the place of origin for Psiloceras, Caloceras, Schlotheimia and 
Wsehneroceras. The migrants that peopled other basins were not the 
degenerate or highly progressive forms of these series, but often, 
and perhaps generally, the radical forms. Thus out of the exten- 
sive series of the Radical Stock, Psiloceras, and of Waehneroceras of 
the Northeastern Alps, but very few species, and these almost 
wholly the radicals migrated to central Europe. The history is 
somewhat different in Schlotheimia, though even in this genus, 
there are many more forms in the basin of the Northeastern Alps 
than in Central Europe. 
There are undoubtedly certain facts, like the earlier appearance 
of fichlot. striatissima in the “gelbe Sandstein” of the Trias in 
Wiirtemburg which do not coincide with the assumption of ori- 
gin of the Schiotheimian series in the basin of the Northeast Alps. 
It ought to be mentioned here also that Psil. planorbe var. levis ap- 
pears in the same basin according to Quenstedt in the Bone-bed, 
which is either the uppermost bed of the Triassic or the very ear- 
liest bed of the Lias. The existence of a bed at the base of the 
