Triasslo. 



Ju^.^SSic. 



C etaceous 



Total, 



34 



120 



217 



371 



245 



1043 



1590 



2878 



393 



49t 



446 



1336 



106 



54t 



509 



1162 



])TSTRTBUT10N OF THE MULLUSCA IN TIMK. 283 



erroneous, believing that the palteozoic forms, have only a 

 superficial resemblance to living forms in their testaceous 

 envelope. They can well suppose that animals essentially differ- 

 ent were able to live in shells of the same form, since in our clays 

 we see analogous facts. For examples, may be cited the genera 

 Carelia, Glandina, Halia, Ferussacia, which were considered as 

 Achatina, until their anatomy became known."* 



Fossil Ilollusks of Mesozoic time. " During the three principal 

 formations of the mesozoic period (Triassic, Jurassic, Cretaceous \ 

 cephalopods of the family Ammonitidse took an extraordinary 

 development, and although the number of species was inferior 

 to that of the lamellibranchs, or even of the gastropods, they 

 nevertheless characterize the most of the stratified beds. Indi- 

 vidually , they were as abundant relatively as were the brachiopods 

 of palaeozoic seas. 



"The following table is prepared from the numbers furnished 

 by Bronn in 1855. 



Brachiopods, species, 

 Lamellibranchs, " 

 Gastropods, " 



Cephalopods, " 



"According to the number of their species the mesozoic Mala- 

 cozoa may be thus classed: 1, Lamellibranchs; 2, Gastropods; 

 3, Cephalopods ; 4, Brachiopods ; 5, Pteropods and Heteropods 

 — scarcely an3^ The mesozoic epoch may consequently be 

 justl}' styled that of lamellibranchs. The predominance of this 

 class of mollusks is nearly constant. 



" If we follow in a determined geographical region, the regular 

 succession of the beds, we will generally find the same relative 

 proportions of the various mollusks. Thus the Liassic deposits 

 of the basin of the Rhone, studied by Diimortier, give us these 

 results : 



*I do not think that Fischer could have selected more unfortunate 

 examples in illustration of this argument. These genera are all readily 

 distinguislitible by the shell alone, and Halia was known to be a marine 

 shell long before we knew anything of the animal. To adduce the eiTors 

 of naturalists, which have arisen solely from carelessness in not noticing 

 manifest characters as a proof that these characters cannot be depended 

 on in fossil genera, when they are the only characters by which we shall ever 

 be able to distinguish them is to change natural history from a science of 

 observation to a speculative science. Fischer covild scarcely have stated 

 more plainly the developmentalist position, than he has done in the above 

 paragraph, the meaning of which is : Since development is true, these 

 ancient appearances of so many modern genera must be deceptive, 

 however plausible. For myself, t prefer fact to theory, and until I can 

 distinguish different generic characters in a palaeozoic Pholadomya, I 

 shall not doubt that it is a Pholadomya. 



