PREVIOUS WORK 423 



(liscrini illation between ^^calcareons argillaceous sliale^' and "argillo-cal- 

 careoiis sliale," foi' example, and applying the term di\ergents to those 

 species wliicli are not constant to single units of this character l)iii (kn i- 

 ate iiiio others, he secures the following figures (page 269) : Of the 766 

 known WeU'h species, 380, or 49 per cent, are divergents; of tlie 167 

 known Wel(;li Brachiopoda, 114, or 6<S jier cent, are divergents; of the 

 841 known New York species, 93, or 11 per cent, are divergent, and of 

 \he 225 known New York Brachiopoda, 18, or 8 per cent, are divergent. 

 He considers the figures for the American area as "an inadequate esti- 

 mate of the true divergence" (page 267), The apparently complete lack 

 of correlation between the different figures given by Bigsby in his ex- 

 haustive compilation would seem to indicate a source of error which is 

 probably attributable to the attempt to carry to so fine a degree of mi- 

 nuteness statements by many different authors as to the nature of the 

 inclosing sediment. Furthermore, determinations based on statements as 

 to the lithologic character of the formation from which a species has 

 been identified have not proven to be dependable, since a shale series may 

 contain thin interbedded limestones filled with their own peculiar fauna. 



Number of Species and Varieties occurring in the different 

 Types of Sediment 



Table I was compiled from a list^ of all the species and varieties of 

 Cambrian and related Ordovician Brachiopoda described in Monograph 

 51 of the United States Geological Survey, giving the number of times 

 each has been identified from each of the three classes of sediment— for 

 example, Lingidella ferruginea was recognized in 25 faunules, of which 9 

 were in limestone, 13 in shale, and j3 in sandstone ; Oholus apollinis was 

 recognized in 23 faunules, all of them being in sandstone ; etcetera, etcet- 

 era. Subtracting from the total number of species and varieties included 

 in this list (579) the species for which we have no data as to the inclos- 

 ing sediment (24), we obtain 555 species, of which 412, or 74 per cent, 

 have been identified from but one type of sediment. This number (412) 

 includes 218 species that have been identified from one locality only, a fact 

 which, under the conditions governing the separation into localities, pre- 

 cludes their identification from more than one type of sediment. The 

 elimination of this number causes the percentage of species identified 

 from but one type of sediment to fall to 35, a figure still sufficiently high, 

 in view of the restrictions under which it was secured, to indicate the 

 pronounced influence of the character of the sea-bottom on the distribu- 

 tion of the brachiopodous species. 



t^ This list covers 25 manuscript pages, and the figures for the individual species do 

 not appear to be important enough to justify its inclusion in this paper. 



