CHAP. xxii. DAVIDSON ON FOSSIL ERACHIOPODA. 427 



character which they exhibited in the earliest formations. 

 On the other hand, other brachiopoda have gone through in 

 shorter periods a vast series of transformations, so that 

 distinct specific, and even generic names have been given to 

 , the same varying form, according to the different aspects 

 and characters it has put on in successive sets of strata. 



In proportion as materials of comparison have accu 

 mulated, the necessity of uniting species, previously re 

 garded as distinct, under one denomination has become 

 more and more apparent. Mr. Davidson, accordingly, after 

 studying not less than 260 reputed species from the British 

 carboniferous rocks, has been obliged to reduce that num 

 ber to 100, to which he has added 20 species either entirely 

 new or new to the British strata; but he declares his con 

 viction that, when our knowledge of these 120 brachiopoda 

 is more complete, a further reduction of species will take 

 place. 



Speaking of one of these forms, which he calls Spirifer 

 trigonalis, he says that it is so dissimilar to another extreme 

 of the series, 8. crassa, that in the first part of his memoir 

 (published some ten years ago) he described them as distinct, 

 and the idea of confounding them together must, he admits, 

 appear absurd to those who have never seen the intermediate 

 links, such as are presented by S. bisulcata, and at least four 

 others with their varieties, most of them shells formerly 

 recognised as distinct by the most eminent paleontologists, 

 but respecting which these same authorities now agree with 

 Mr. Davidson in uniting them into one species.* 



The same species has sometimes continued to exist under 

 slightly modified forms throughout the whole of the Lower 

 and Upper Silurian as well as the entire Devonian and Car 

 boniferous periods, as in the case of the shell generally known 



* Monograph on British Brachiopoda, Paleontological Society, p. 222. 



