﻿268 



T. Davidson — What is a Brachjiopod ? 



comparative anatomy, so essential to the proper -understanding of 

 fossil, or extinct genera and species. 



But to return to our subject. I cannot do better than to extract 

 from Lyell the following passage. " Nothing is more remarkable, 

 in the Silurian strata generally of all countries, than the pre- 

 ponderance of the Brachiopocia over other forms of Mollusca. Their 

 proportional numbers can by no means be explained by supposing 

 them to have inhabited seas of great depth, for the contrast between 

 the palseozoic and the present state of things has not been essentially 

 altered by the late discoveries made in our deep-sea dredgings. We 

 find the living Brachiopoda so rare as to form about one forty-fourth 

 of the whole bivalve fauna, whereas in the Lower Silurian rocks, and 

 where the Brachiopoda reach their maximum, they are represented 

 by more than twice as many species as the Lamellibranchiate bi- 

 valves. There may, indeed, be said to be a continuous decrease of 

 the proportional number of this lower tribe of Mollusca as we pro- 

 ceed from the older to the newer rocks," 



Dr. Bigsby informs me that from a conspectus in his new The- 

 saurus (now in the press) the following numbers o-f Silurian, Devo- 

 nian, and Carboniferous Brachiopoda have been given, with due re- 

 gard to accuracy, but that after all it is only a careful approximation 

 liable to future disturbances : 





America. 



EuTope, etc. 



Total. 



Cambrian and Silurian 



689 



577 

 488 



738 



789 

 384 



1422 



Devonian 



1366 



Carboniferous , 



871 









1754 



1905 



3659 



The number fro^m the Permian formation, which completes the 

 PalaBOzoic series, has not yet been computed, but they are compara- 

 tively few in number. Making a large allowance for synonyms, it 

 will be seen that fully 3000 species are already known to have 

 existed during the primary periods. It is also a remarkable fact that 

 the Brachiopoda, so immensely abundant during the Cambrian, 

 Silurian, Devonian, and Carboniferous periods, became apparently 

 very much less numerous during the Permian and Triassic ; while 

 they again became abundant, although comparatively reduced in num- 

 ber, during the Jurassic and Cretaceous period. In the Tertiaries 

 they had materially decreased in number, and are represented, at the 

 present time, by about one hundred species. It has also been clearly 

 ascertained that a certain number of genera and species passed from 

 one system or formation into the one that followed it, as may be 

 seen by a glance at the Table, in which the general distribution of 

 the genera in time has been given with as much accuracy as the 

 present state of our knowledge will admit. From this table it will 

 be perceived that about 9 genera appeared for the first time in the 

 Cambrian system, 52 in the Silurian, 21 in the Devonian, 7 Carbon- 



