CHAP. XXII. DAVIDSON ON FOSSIL BRACHIOPODA. 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 dis- 

 tinct s^Decific 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 accumu- 

 lated, the necessity of uniting species, previously regarded 

 as distinct, under one denomination has become more and 

 more apparent. Mr. Davidson, accordingly, after studying 

 not less than 260 reimted species from the British carbo- 

 niferous rocks, has been obliged to reduce that number to 

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

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

 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 tri- 

 gonalis, 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 dis- 

 tinct; 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 recognized as distinct by the most eminent 

 palaeontologists, 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 



*■ Monograph on British Brachiopoda, Palwoutological Society, p. 222. 



