930 Report or THE STATE GEOLOGIST. 
son for assuming that the growth of the brachia at their 
extremities, which produced the median arm, was necessarily 
discontinued, but rather that this median unpaired arm coexisted 
with the lateral paired spirals. This course of argument, though 
seemingly logical, appears to be be based on insufficient premises. 
The brachiopods with which we have to deal in the Paleozoic are 
essentially primitive structures, whether rhynchonellids, tere- 
bratuloids or spire-bearers. If the living RuyncHoneLia and 
TEREBRATELLA possess in their mature condition extensive 
free arms, it does not necessarily follow that their early 
palxozoic 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 develop- 
ment in the AncyLopracuia 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 primitive terebratuloids, as CenTRo- 
NELLA, RensseL@rta, Crypronetya, Dretasma, etc.; in TrRopt- 
DOLEPTUS, Which has been shown to represent a highly primitive 
phyletic condition of the TerzpraTeLLip#; and, also in the 
earliest spire-bearers and rhynchonellids. Hence the conclusion 
above expressed as to the successive phyletic relations of the primi- 
tive rhynchonellids, terebratuloids and spire-bearers and based upon 
the relations and modifications in the form of their brachial sup- 
ports, is fairly substantiated by the evidence drawn from other data. 
The divergence from the ancestral rhynchonellid stock was very 
early and the differentiation undoubtedly consisted, toa large extent 
in rapid acceleration of growth in the brachia, and obstruction to 
the coextensive development of the fleshy arms and their supports- 
Finally, it is desirable to again recall the intimate similarity 
between RenssELeria and the pentameroid genus AMPHIGENIA ; 
genera in which the essential distinction between the typical 
forms of each lies in the simple loop of the former and the long, 
expanded but still discrete crural processes of the latter. Atten- 
tion has been directed to these similarities and differences, and it 
has also been pointed out that the spondylium in Amphigenia 
elongata is at times almost reproduced in specimens of Rensseleria 
ovoides where the dental lamellz are highly developed. 
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