94 GENESIS OF THE ARIETID^E. 



The radical form Sohlot. catenata appeared in the Planorbis horizon, according 

 to the collection at Semur, and in this fauna the successive forms of the schlo- 

 theimian series succeed one another without a break in their gradations. Quen- 

 stedt's work on " Die Ammoniten des Schwabischen Jura " shows that in South 

 Germany the series may be complete in numbers of forms, and even more re- 

 markable in the size of specimens, and the whole series except Boucaultianus 

 appeared before the termination of the Angulatus fauna. There is also a speci- 

 men referred doubtfully even to the " gelbe Sandstein " of the Rha?tic beds near 

 Tiibingen, thus carrying the possible origin as far back as in the Northeastern 

 Alps. If, as we have supposed, Wcehn. subangulare is found in South Germany, 

 the evidence becomes still stronger that this was the probable centre for the 

 chorological distribution of the group in Central Europe. 



Schlot. angulata and catenata are very numerous in North Germany ; but there 

 is a notable tendency to the production of smaller specimens in the collections 

 we have seen. Schluter, in his " Schichten des Teutoburger Walcles bei Alten- 

 becken," 1 states that Amm. angulatus, Moreanus, and Gharmassei occur there, but 

 that the latter is never so large as in South Germany. 



Terquem, in his "Province de Luxembourg et de Hettange," 2 mentions Amm. 

 angulatus as occurring in abundance, and of good size, but no species like Char- 

 ■massei or Leigneletii. Chapuis and Dewalque give figures of Schlot. angulata, 3 which 

 show that the species is similar to catenatus in having discoidal whorls and 

 the pilae crossing the abdomen. This species is evidently similar to that from 

 Markoldendorf, described by Emerson. Seebach, in his " Hannoverische Jura," 

 declares that in North Germany there has so far been found only the Amm. angu- 

 latus (equal to depressus, catenatus, and Moreanus), and denies the existence of 

 Gharmassei. Brauns, in his " Untere Jura im nordwestlichen Deutschlands," cites 

 both angulatus and Gharmassei. 



The whole series, including radical, discoidal, and involute species, appear to 

 have come into the Northeastern Alps basin first, and to have reached in this 

 locality their highest development in discoidal forms. Thence they seem to 

 have spread somewhat later in time into the Angulatus horizon of the South 

 German basin, and migrated still later to the C6te d'Or and England. In the 

 first two basins they reached their highest development in involute forms, — 

 a fact which strengthens the impression that the series must have originated in 

 the Mediterranean province, since the involute forms are the descendants of the 

 discoidal forms. That they arrived in the Cote d'Or later than in South Ger- 

 many is shown by Tables I. and II., in which we find Gharmassei appearing in the 

 Lower Bucklandi zone instead of the Angulatus zone, and by the presence of 

 two new and more highly modified species, D ' Orbigniana and Boncaultiana, which 

 have not been found in South Germany by any collector up to the present day. 

 These views are further sustained by the fact that the English fauna possesses 

 only a slender representation of the group, all the species being rare, and occur- 

 ring at about the same time as in the Cote d'Or, except Boncaultiana, which is 



1 Page 42. 2 Mem. de la Societe Geol. de France, V. 



3 Descrip. Foss. Terr. Secern, de Luxembourg. 



