A. PAVLOW MARINE MESOZOIC FORMATIONS. 63 



cus, B. parrectus, etc. I add to them some other species and designate that branch 

 by the name of Porrecti. It commences in the Callovian by smooth forms without 

 ventral groove ; in the Oxfordian and the Kimmeridgian we find the same elon- 

 gated form with a short ventral groove below ; while in the upper Kimmeridgian 

 these belemnites have a ventral groove which passes from one end to the other. 



The second Branch comprises the thicker and less conical forms. It begins in the 

 Callovian by Belemnites spicularis, a form almost devoid of a groove, which in the Ox- 

 fordian gives rise to B. oiveni. The latter is gradually transformed into B. magnifieus, 

 in which the groove or ventral flattening is very distinct and occupies about one-half 

 of the rostrum (guard). All the belemnites mentioned are common in Russia and in 

 England in the successive beds from the Callovian to the Kimmeridgian. B. mag- 

 nifieus gives birth to B. absolutus, the culminating form, which is widely spread and 

 very common in the upper beds of the Jurassic of Russia. I designate this branch 

 by the name of Magnifici. 



The third branch, which I call Explanati, is the most complicated. Starting from 

 a Callovian branch, Belemnites subextensus, we see three sub-branches (twigs) devel- 

 oping each in its own direction. One comprises the thick-set and obtuse forms (B. 

 kirghisensis, B. lateralis and B. russiensis), while the other begins by B. breviaxis, 

 which is modified into B. e.rplanatus, and this in turn passes into B. subquadratus 

 and a kindred species, B. explanatoides. The third sub-branch begins by B. panderi, 

 which is transformed into B. troslayanus, the predecessor of B. mosquensis. I am 

 now convinced that B. panderi, and perhaps some allied species, exist in America, 

 where they are known under the name of B. dermis. As in the old world, so in 

 America, they characterize the boreal provinces of the Jurassic sea. The history of 

 the development of this sub-branch is the most interesting. In northern England 

 these forms are developed continuously up to a certain horizon, namely, the summit 

 of beds D, called Portlandian, but which are also considered by some geologists as 

 lower Neocomian. Above this horizon, in the beds C, these forms disappear 

 abruptly, and are replaced by belemnites of quite a different origin, B.pistxMrosbris, 



B. jaculum, and other representatives of Hastati, which appear simultane* >usly. Mr. 

 Lamplugh, during several years of assiduous research at Speeton, found only two 

 specimens belonging to the preceding group. But already in the upper part of beds 



C, and above these beds, the Hastati become less and less numerous, and we find 

 once more the belemnites exhibiting the characters of the Jurassic group Explanati, 

 but they are the more or less distant descendants of the Jurassic forms. 



The Explanati were evidently dwellers in the boreal part of the Jurassic sea. They 

 are known in Russia, in northern England, in North America (Queen Charlotte 

 islands and Dakota). They are also found in France and in southern England, but 

 they are rarer in those regions. The Hastaii are the southern forms. They are 

 wide-spread in the Alps, in southern Europe, in the Caucasus, in India, and in 

 Madagascar. Thus we observe at Speeton, at a certain horizon, the invasion of the 

 southern fauna in the northern sea, and the replacement of the boreal fauna by the 

 southern fauna. But the predominance of the southern fauna was nut of long du- 

 ration ; already in the upper Neocomian, and perhaps also in tin- middle, conditions 

 changed, and the descendants of the boreal forms come to regain the dominion of 

 their ancestors. In Russia the history of the faunas is less complicated, because 

 the southern colony did not exist there, except in the Crimea. 



The history of the belemnites which 1 have just set forth is only an example 

 affording us a glimpse into the history of the Mesozoic seas at the epoch in question. 



