BRACHIOPODA. 
U\) 
discontinued, but rather that this median unpaired arm coexisted with the 
lateral paired spirals. This course of arj^ument, though seemingly logical 
appears to be based on insufficient premises. 
The brachiopods with which we have to deal in the palaeozoic 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, PtENSSELAERiA, Crypto- 
NELLA, Dielasma, etc.; in Tropidoleptus, which has been shown to represent a 
highly primitive phyletic condition of the TuREBRATELLinyE; 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. 
