BRACHIOPODA. 319 



S U M NI A K Y. 



THE EVOLUTION OF THE GENERA OF TFIE TAL^-OZOIC BRACHIOPODA. 



At the conclusion of the discussions upon the Inarticulate palaeozoic genera, 

 some inferences were drawn as to the phylogeny and derivation of the more 

 conspicuous types of inarticulate structure (Part I, pp. 161-170). At that time 

 it had become evident that the variation in the form, position and mode of 

 enclosure of the pedicle-passage affords a more satisfactory index of lines of 

 progress and development, and gives a more lucid and reliable conception of 

 the rise and decline of brachiopod genera, than the modifications in any other 

 single character or association of characters. 



Previous writers have usually ascribed a high value to the disposition of the 

 muscular scars upon the inner surface of the valves, the form of the genito- 

 v<ascular sinuses, the configuration and degree of calcification of the brachia. 

 The last of these must still be regarded as having a significance inferior in 

 importance only to the mode of enclosure of the pedicle; but to the other 

 features mentioned our present knowledge accords a less value in classification. 

 By this is meant that the muscular system, the disposition and interrelations 

 of the separate muscular bands, adheres closely to a standard type of expres- 

 sion throughout the Class. 



This is especially true of the Articulate genera, where, from beginning to 

 end, no radical modification of the type, in this respect, is effected. It is less 

 true, perhaps, in the more highly specialized and more complicated muscular 

 structures of the inarticulates, a group in which our knowledge of the fossil 

 representatives is not altogether satisfactory on account of the tenuity and 

 ready destructibility of the shells. It is quite natural to find in such a highly 

 organized group the possibility of variation more frequently manifested. 



The opinion expressed in the "Conclusion" to the Brachiopoda Inarticulata, 

 that the " feature of paramount importance " in dealing with the evolution of 



