JIG GENESIS OF THE ARIETIDJC. 



Spezia, and another higher horizon, which is supposed to be the equivalent of the 

 Bucklandi horizon. This last is said to contain Cephalopods representing all the 

 later faunas of the Lower Lias, and some species are quoted as being referable to 

 the Middle Lias. 



Canavari, in his " Fauna der unteren Lias von Spezia," so frequently quoted 

 above, states that the fossils occur in a single zone, which does not admit of sub- 

 division, though it was carefully investigated, layer by layer, by Cocchi. He 

 states also, that it is unquestionably the lowest of the lower lias sediments in 

 Italy, and comprises all the horizons except those of Planorbis and Oxynotus. 

 He considers that the fossils have closer affinities with those of the Mediterranean 

 province than with those of Central Europe, a fact which seems to be established. 

 The species of the Lytoceratidas and of Amaltheus, etc., which are supposed 

 to be anachronic and to indicate a fauna derived from the Middle and Upper 

 Lias, appear to us to be found in their appropriate positions, like those of the 

 Northeastern Alps. They may be either the radicals of the similar forms which 

 occur in the Middle and Upper Lias of the Central European faunas, or morpho- 

 logical equivalents, or pathological specimens. 1 This is also Canavari's opinion 

 with relation to some forms, since he expressly states that the segoceran species, 

 as he calls them, are the immediate forerunners of Microderoceras. The fauna 

 of the Rhone basin is almost exclusively composed of species having a Central 

 European aspect. There are, it is true, some slight indications, in the presence 

 of three species of Lytoceratidae in this basin, that the migrants may have come 

 this way on their march into Central Europe, but there are no supporting facts 

 with which we are acquainted. The absence of the Planorbis horizon, or at any 

 rate its sporadic appearance in Italy, and the absence of Annnonitinaa in this 

 horizon of southern Provence, are very serious difficulties in the path of a sup- 

 posed southern track of migration. 



The evidence, so far as known, seems therefore strongly in favor of the view, 

 that during the time of Planorbis and Caloceras, and perhaps earlier in the 

 Angulatus horizons, the stream of migration flowed south and westerly from the 

 Northeastern Alps into Italy, while another from the same basin directed itself 

 westerly along the then existing coast lines into the basins of South Germany 

 and the Cote d'Or, and the species were distributed thence into the basins to 

 the north and south of these two. in the province of Central Europe. In South 

 Germany and the Cote d'Or the conditions became favorable during the time of 

 the Angulatus horizon for the evolution of Vermiceras among the descendants of 

 the Plicatus Stock, and for the origin of Coroniceras, Arnioceras, and Agassiceras 

 ot the Levis Stock. Asteroceras arose later in these same faunas in the Upper 

 Bucklandi beds, and Oxvnoticeras probably even still later, though here the 

 series is evidently older than the date of its first appearance. The migrations of 

 these genera, spread the forms to the east into the faunas of the Northeastern 



1 The figures of Amaltheus given by Canavari in his last work. "Fauna del Lias inf. della Spezia, 

 K. Comit. GeoL .1' Italia," III., l't. IT. pi. vi., are certainly startlingly similar to Amaltheus, but such 

 resemblances in forms of widely distinct series are not uncommon. Sec the pathological case figured on 

 Plate X. Fig. 19 of this memoir, an, I others quoted in the descriptions of the species. 



