BRACHIOPODA. 
319 
S U JVC jVE A R Y. 
THE EVOLUTION OF THE GENERA OF THE PALHtOZOIC 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- 
vascular 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 
