266 GEOGRAPHICAL A^D GEOLOGICAL DISTEIBUTION. 



ent determined; it would appear at first sight as though the length 

 of its duration were sufficient, for how otherwise could we in- 

 telligently explain the total or nearly total absence of the members 

 of at least two of the molluscan group from deposits in which the 

 representatives of other groups are sufficiently abundant ? One 

 fact, however, must not be lost sight of in this connection, and 

 that is, that in these earliest deposits the obliteration of organic 

 remains has been most excessive, and that not improbably the 

 absence of the required forms is to be attributed rather to the 

 destruction of parts than to an actual non-existence in the region. 

 For the present it is impossible to affirm whether the Cephalophora 

 came into existence before the Lamellibranchiata or not, but the 

 evidence scarcely appears sufficient for considering the latter as a 

 race derived by degeneration from the former, as has been pre- 

 sumed by Professor Lankester. 



The sudden decline of the Cephalopoda (Nautilidse) after the 

 close of the Silurian period is very remarkable, and scarcely less 

 so their rehabilitation under the form of their successors, the Am- 

 monitidse, in the deposits of the Mesozoic era; Barrande enumerates 

 but two hundred and forty-two species from the Devonian forma- 

 tion, more than one-half of which belong to the genus Orthoceras, 

 and the remainder principally to the genera Cyrtoceras, Gyroceras, 

 and Gomphoceras. About an equal number are indicated from the 

 deposits of Carboniferous age, where also we find much the same 

 genera represented, although with different specific relations. The 

 genus Nautilus now for the first time acquires any importance, and 

 it and Orthoceras alone of the linuted surviving members of the 

 family of the Permian period transgress the boundaries of the 

 Paleozoic era. The latter genus disappears early in the Trias, 

 while the former steadily increases in number, imtil in the Creta- 

 ceous deposits it attains its maximum development, with a repre- 

 sentation of some sixty or more species. 



Tlie displacement of the Nautilidse by the Ammonitidse is, if 

 nothing more, certainly an interesting circumstance, and leads one 

 to inquire what special advantage the latter may have possessed 

 over the former in the struggle for existence, by means of which 

 they triumphed over their predecessors. For there can be little 

 question that the Ammonitidse, despite certain peculiarities in their 

 structure, which are not as yet comprehensible to us, are the truly 



