THE BRACHIOPODA 23 



identical with it by their minor features. Hence the 

 apparent paradox that it is often easier to determine the 

 species of a fossil than its genus, which is quite contrary 

 to the ordinary conception of classification. For the 

 specific character is commonly some feature of external 

 shape or ornament that is easily recognized but might be 

 found in any one of a number of genera, while the generic 

 character may be some feature which only an excep- 

 tionally well-preserved specimen will show. We must 

 never forget that classifications are not framed as means 

 for the easy naming of specimens, but are intended to 

 indicate the real natural relationships of species. 



In the particular case of the internal characters of 

 brachiopods, however, these are not so inaccessible as 

 a beginner may think. It often happens that the interior 

 of a shell may be filled, after death, with foreign material 

 such as mud or sand, and that afterwards the shell may 

 be dissolved, leaving the consolidated mud or sand as in 

 internal cast or mould of the shell. On such a cast all 

 the structures on the inner surface can be recognized 

 hollows being represented by elevations, and projections 

 by depressions. It is even advisable, when the infilling 

 material is suitable and specimens are abundant, to make 

 casts artificially by burning away or otherwise removing 

 the shell. 



ORDER I. : ATREMATA. 



Inarticulate Brachiopoda with horny and phosphatic 

 shells, the pedicle emerging between the divergent valves, 

 which show little or no tendency to enclose it in a 

 delthyrium. 



