118 GENESIS OF THE ARIETIDA. 
the great abundance in which Ammonoids occur as contrasted with Nautiloids, 
are all in favor of the conclusion that they were structurally rostrated, creeping 
animals, which necessarily followed the shore lines in their migrations. Fraas 
takes the view that the Suabian Lower Lias was as a whole, when compared 
with the synchronous strata to the west and north, a deep-sea formation, and 
cites the absence of sandstones and coarse deposits, the small Lamellibranchs, 
and Brachiopoda. It is very evident, however, that whatever the bathymetrical 
differences of the South German basin, and however far removed from the 
ancient shores now represented by the Black Forest and the Vosges, the sur- 
roundings were not sufficiently distinct to make any marked differences in the 
Arietidee of this basin. 
We have noted above the occurrence in Peru of Cal. Ortoni, a form having 
close resemblance to a species of the Northeastern Alps, and the apparent iden- 
tity of other species with those of Central Europe, the forms found at Vancouver's 
Island and in California, etc., show that on this continent the faunas possessed a 
mixed character. The paucity of the development both geologically and paleon- 
tologically of the Lower Lias is in accord with the similar deficiency of this stage 
in the analdainic basins of northern Europe, India, and Italy. There is another 
fact in this connection, which strikes us as very remarkable, — the absence of any 
absolutely new types of Ammonitine. So far as explorations have gone, not a 
single species indicates the evolution of any widely distinct family or genus from 
those found in Europe. Thus, although not able to produce any satisfactory evi- 
dence that all the faunas throughout the world during the lower lias age were 
more or less analdainic faunas derived from the zone of the autochthones of the 
Arietidxe in Europe, the evidence is sufficient to make such an opinion worthy of 
the attention of students of geology and paleontology. 
The view expressed by Neumayr, that the Cephalopoda are exceptional 
in respect to the rapidity with which their modifications probably took place, 
seems to us erroneous. ‘here is no greater aspect of pliability in this than 
in other types, when accurately classified. When, however, we assemble 
within the same family species of the Lytoceratinse and Ammonitins, or in 
the same genus forms of entirely distinct stocks without sufficient reference 
to their genetic history, then of course a belief in the polygenesis of the 
progressive series,’ and in an exceptional tendency to modification, becomes 
essential in order to explain the heterogeneous aspect of the groups. We think, 
however, that even the most variable families of Cephalopoda are not, as a 
rule, any more variable than the Unionidae, Ostreadse, or Hippuritidee, among 
Lamellibranchs, or the Planorbidew, Vermetide, etc., among Gasteropoda, and 
many other groups that might be mentioned. 
The expansion of the whole series of forms of Psiloceras, Schlotheimia, and 
Weehneroceras in the Northeastern Alps, and the apparent rapidity of chorologi- 
cal migrations and changes and introduction of new series, the equally sudden 
1 We desire to call attention here to the fact that we have admitted the polygenetic derivation of retro- 
gressive types like Baculites, etc.; but this in no manner commits us to the doctrine of polygenesis for any 
of the progressive types. So far as we know, these are monogenetic in mode of origin. 
