112 GENESIS OF THE ARIETIDiE. 



Rhone. The last two are closely comparable in aspect with species figured by 

 Hauer from the Adneth limestones. 1 In the same way, we should be disposed 

 to regard the Mediterranean province as the autochthonous home of some genera 

 of the Middle Lias, which appear here in association with the Arietidae. 



The Arietidae afford an excellent standard, since their genera and species 

 have been found, with rare exceptions, only in the Lower Lias; and, so far as 

 our knowledge now goes, the series of forms and cycles have a very complete 

 and satisfactory aspect, indicating a history of progress and decline within the 

 limits of that group of strata in the faunas of Central Europe. 



In the Mediterranean faunas, however, so far as known, only the rise of the 

 group is recorded in the sediments and fossil remains, and its acme and decline 

 are not clearly indicated. We have been accustomed to look upon the fauna of 

 the Ilierlatz beds as composed for the most part of degraded dwarfs, whose pecu- 

 liarities or modifications were due to the unfavorable action of the surroundings 

 upon migrants from other contemporaneous faunas of the Lower Lias. This 

 seems to be the only theory which can account for the prevalent smaller size 

 and more or less degraded aspect of many of the shells, when compared with 

 their nearest allies in other locations. 



Summary. 



The facts cited above, though far from complete, show that the series of the 

 Radical and Plicatus Stocks, with the exception of the vermiceran series, were 

 probably evolved in the Mediterranean province. The series of the Levis Stock 

 had however a different history, since they probably arose in the basins of Cen- 

 tral Europe. We therefore venture to differ in part from the eminent geologist 

 and paleontologist Neumayr, who regards, if we properly understand his views, 

 the Northeastern Alps as the aldainic home of the whole of the Arietidae. 



The sutures of all the Mediterranean forms of Psiloceras and Caloceras are, as 

 figured by Wahner, more complicated, or, as we should say, more triassic than 

 those commonly found in Central Europe ; but we occasionally find a variety of 

 Psil. planorbe, like that figured by Quenstedt 2 and by Wright, 3 in which there is 

 a close approximation to the outlines common in the Mediterranean province. 

 After having written the above, Ave were extremely gratified to find precisely the 

 same results with regard to the relation of caliphjllum and planorbe, but more fully 

 and exactly stated by Neumayr, in his " Unterster Lias" (p. 25). His conclu- 

 sion, that planorbe is consequently a derivative of Psil. calipliyllum, and is char- 

 acteristic of Central Europe, while the latter species is equally characteristic of 

 the Mediterranean province, is sustained by the fact that the sutures of caliphjl- 



1 The peculiarities of the senile whorls are similar to those of Oxynoticeras Lotharingwn, and will lead 

 to much confusion until the sutures and the young are fully known. It is quite possible that our own con- 

 clusion may be wrong in this respect, but the sutures of Salisburgensis and alius, Hauer, are Lytoceran, and 

 the aspect of these compressed shells is very similar to that of those found in France, whose sutures are 

 however unknown. The young are known only in Driani, which resembles some of the forms described 

 by Herbich. 



2 Amm. d. Schwab. Jura, pi. i. fig. 19. 3 Lias Amm., pi. xiv. fig. 1. 



