SUMMARY 433 



The results do not necessarily prove that a species is to be considered 

 as generally confined to one type of sediment ; it may have been collected 

 from shale only, for example, when it may also occur in sediments of 

 entirely different character, but of identical age, in which it has not yet 

 been discovered. Neither can the single fact that two beds of unlike 

 lithologic characters bear different faunas be advanced as an indisputable 

 argument against their contemporaneity. Certain it is that modern 

 brachiopods appear to be able to exist at widely differing depths without 

 observable modification in structure, and we know of apparently identical 

 fossil forms which have preserved their individuality through lateral as 

 well as vertical changes in tJie character of the inclosing sediment. We 

 should be justified, however, in saying that the evidence at hand for the 

 Cambrian and related Ordovician Brachiopoda points largely to a more 

 or less positive influence of the character of the sea-bottom on both the 

 nature and the number of the inhabiting species. 



Summary 



A careful study of the known Cambrian and Lower Ordovician brach- 

 iopod localities considered in this paper shows (a) that the 47 genera, 

 16 subgenera, 519 species, and 60 varieties represent 1,460 localities in 

 16 countries and 5 continents; (h) that from about 72 per cent of the 

 localities represented in the United States National Museum brachiopods 

 have been identified; (c) that, dividing the sediments into three groups 

 (shale, sandstone, and limestone), 41 per cent of the genera and sub- 

 genera and 74 per cent of the species and varieties appear to have been 

 identified from but one type of sediment;^ (d) that of the 26 genera and 

 subgenera which appear to be confined to one type of sediment only 14 

 (or 22 per cent of the total number of known genera) have been identi- 

 fied from more than one faunule, but that of this number ten (or 71 per 

 cent) are confined to sandstone; (e) that 44 per cent of the species oc- 

 curring in more than one type of sediment have been identified from 

 more than one of the three main divisions of the Cambrian ; (/) that of 

 the species common to differing types of sediment, 63 per cent have been 

 collected from more than one State or similar artificial unit, and that of 

 this number 55 per cent are also common to separable geologic provinces ; 

 (g) that the species occurring more than once in the same section largely 

 (45 out of 70) accommodate themselves to vertical changes in the char- 

 acter of their inclosing sediment, though an appreciable number (14) 



^ These figures (41 and 74) become reduced to 22 and 35 per cent respectively by the 

 elimination of (a) genera represented by single species in single faunules and (b) species 

 represented by single localities. 



