110 GENESIS OF THE ARIETIDA. 
Mediterranean province are summarized by Wiihner in this very satisfactory 
paper, and one sees that the lowest beds are apt to be well defined, but that 
after passing through the Angulatus zone definition becomes more difficult, so 
that even this author, for whom as an acute discriminator of species we have a 
great respect, seems not to have been able to define the separate beds in either 
the Adneth or the Hierlatz limestones. 
Herbich makes a valuable contribution to this problem in his Széklerland, 
in which he describes several species of the Arietide, including an Asteroceras 
like stellaris of Hauer, equivalent to our obtusum, var. stellare, and Atgoc. Althii, 
which appears to be a true Microceras allied to Micr. planicosta, together with a 
number of species of the Lytoceratidse, all occurring in a bed not over three 
meters thick, and he denies that any distinct beds can be defined! Geyer, 
in the work above quoted, gives a detailed argument for the probable 
admixture of faunas, and comes to the conclusion that Oppel’s scheme of 
zones is not applicable to the Northeastern Alps so completely as it is to the 
formations of Central Europe. Favre, in his “Terrains Liassiques et Keupe- 
riens de la Savoie,’ gives a list of localities in which mixtures of different 
faunas have been announced by various authors, and Geyer adds several other 
localities. 
Favre considers that the species in such localities, among which he includes 
the Northeastern Alps, must have been protected from the geological changes 
which produced new forms and modifications in other localities, and adds that 
we must seek the causes of admixture in the continuation of sediments of the 
same nature, and in the configuration of the surface. His idea was, that the per- 
sistent species continued to exist in closed basins, where they were secure from 
the action of the causes that destroyed the faunas to which they originally 
belonged in other localities. This explanation has a reasonable sound, but it 
appears to us inadequate. We regard the species quoted as migrants from pre- 
viously existing faunas, which, having found favorable homes in these localities, 
became the radicals of new series upon new horizons; or else they were survivors 
of the geratologous forms of faunas upon the same horizon, which, having found 
favorable conditions in these new localities, persisted perhaps somewhat longer 
than the parent series. We have not found adequate evidence of closed areas, 
except perhaps between the western extension of the Mediterranean province as 
a whole, and that of Central Europe. The basins of the Lower Lias were evi- 
dently not, as a rule, so completely closed as to keep out migrants from other 
basins and provinces, since all the evidence tends to prove the constancy and 
uninterrupted migration of species throughout the faunas of Central Europe and 
the Mediterranean province. 
Whatever hypothesis is maintained, there seems to be no possible way of 
accounting for the finding of a species in a truly anachronic position ; that is to 
say, in a bed which belongs to an earlier horizon than that in which it has been 
proved to have originated. A specimen of Coroniceras Bucklandi in the Planor- 
bis bed, or even in the lower part of the Angulatus bed, would introduce great 
1 Page 108. 
