DISTRIBUTION OF THE MOLLUSCA IN TIMK. 231 



Fossil MoUusks of the Palseozoic Period. 



•' The study of the earliest manifestations of life upon the 

 surface of the globe will alwa3'S have great attraction for 

 naturalists. In 1868, Bigsby prepai-ed the following table of 

 the primordial or Cambrian fauna, which includes 912 species. 

 Plantse, 22 Asteridea, 1 Brachiopoda, 116 



Amorphozoa,27 Annelida, 29 Lamellibranchiata, 12 



Coelenterata, 6 Trilobita, 411 Pteropoda, 5*7 



Crinoidea, 1 Entomostraca, 25 Gastropoda, 115 



Cystidea, 2 Polyzoa, 77 Cephalopoda, 65 



" Consequently, from the beginning the principal types of 

 mollusca are represented ; the gastropods are as well developed 

 as the brachiopods, and the cephalopods surpass in number the 

 pteropods, which are, ncA^ertheless, inferior in organization. In 

 the middle and upper beds of the Silurian, the brachiopods make 

 an enormous increase ; the cephalopods arrive at their apogee ; 

 then come, according to their importance, the gastropods, the 

 lamellibranchs, and iinally the heteropods and pteropods. 



" In the Devonian and Cai'boniferous periods, the brachiopods 

 sensibly diminish, and lose the first place, which is occupied hj 

 the lamellibranchs ; the gastropods become more numerous than 

 the cephalopods. 



Brachiopods, species, 

 Lamellibranchs, species, 

 Gastropods, species, 

 Pteropods and Heteropods, species, 

 Cephalopods, species, 



" Consequently, in palseozoic periods the classes of Malacozoa 

 occupy the following order according to the number of their 

 species: 1, Brachiopods; 2, Lamellibranchiates ; 3, Cephalo- 

 pods ; 4, Gasti'opods ; 5, Pteropods and Heteropods. 



" We can then characterize the palseozoic epoch as the age of 

 brachiopods. 



" We have no table of mollusks of the Permian, but it is 

 scarcely probable that the fossils of this fonnation would modify- 

 the preceding conclusions. 



" The Nautilidse, among the cephalopods, shone with all their 

 lustre during the Silurian ; it is then that their genera presented 

 the most varied forms. Orthoceras was multiplied to a degree 

 unheard of, since in the single basin of Bohemia, Barrande has 

 been able to distinguish 554 species. 



" There lived in the Silurian seas at the same time some pelagic 

 mollusks (pteropods and heteropods), belonging to the genera 

 Bellerophon, Conularia, Maclurea. The gastropods were nearly 

 all holostomate : Acroculia, Euomphalus. Loxon6ma, Murchi- 

 sonia, Pleurotomaria, Platyostoma. The principal lamellibran- 



Silurian. 



Devonian. 



Carbonif. 



Total. 



1650 



695 



875 



3220 



709 



927 



1214 



2850 



895 



621 



674 



•2190 



., 338 



138 



108 



604 



1454 



558 



410 



2424 



