BRACHIOPODA. 349 



discontinued, but rather that this median unpaired arm coexisted with the 

 lateral paired spirals. This course of argument, though seemingly logical 

 appears, to be based on insufficient premises. 



The brachiopods with which we have to deal in the palasozoic are essentially 

 primitive structures, whether rhynchonellids, terebratuloids or spire-bearers. 

 If the living Rhynchonella and Terebratella in their mature conditions possess 

 extensive unsolidified arms, it does not necessarily follow that their early 

 palaeozoic representatives were provided with similar uncalcified extensions ; 

 on the contrary, it would be much more reasonable and in accordance with our 

 knowledge of natural laws to infer that in these early forms the adult condition 

 of the brachia was more nearly that of immature conditions of these organs in 

 their living representatives. There is a primitive condition of development in 

 the terebratuloids in which the loop is coextensive with the brachia; there is 

 reason to believe that such has been the relation of these parts in the mature 

 phases of the primitive terebratuloids, as Centronella, Rensselaeria, Crypto- 

 NELLA, DiELASMA, ctc. ; in Tropidoleptus, which has been shown to represent a 

 highly primitive phyletic condition of the Terebratellidjs ; and, also, in the 

 earliet spire-bearers and rhynchonellids. Hence the conclusion above expressed 

 as to the successive phyletic relations of the primitive rhynchonellids, terebra- 

 tuloids and spire-bearers, and based upon the relations and modifications in the 

 form of their brachial supports, is fairly substantiated by the evidence drawn 

 from other data. 



