110 GENESIS OF THE ARIETID^E. 



Mediterranean province are summarized by Wahner 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 Szeklerland, 

 in which he describes several species of the Arietidse, including an Asteroceras 

 like stdlaris of Hauer, equivalent to our obtusum, var. stellare, and JEgoc. AUhii, 

 which appears to be a true Microceras allied to Micr. planicosta, together with a 

 number of species of the LytoceratidsB, all occurring in a bed not over three 

 meters thick, and he denies that any distinct beds can be defined. 1 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, 

 exei'pt 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 103. 



