348 
PALEONTOLOGY OF NEW YORK. 
must long be a matter of legitimate speculation, and in view of this fact a few 
arguments of such a nature in this place will be permissible. The living 
representatives of Rhynchonella and Terebratula are animals in which a very 
considerable part of the brachia does not become sufficiently spiculized to form 
a continuous calcareous support. In R. {Hemithyris) psittacea, for example, the 
brachia are as highly developed in the form of coiled spiral arms as they could 
have been in most of the ancient spire-bearers, but their calcareous supports 
are only the short lamella) known as the crural processes. All of the living 
Ancylobrachia which possess a long curved loop like that of Cryptonella and 
Dielasma of the Palaeozoic, have an unsupported median unpaired spiral arm, 
coiled ill a direction which is the reverse of that prevailing among the spire- 
bearers If, now, we are to interpret the condition of the brachia in their 
nearest living representative, it becomes necessary to assume that on the one 
liand, the palaeozoic rhynchonellids possessed long colled spiral arms, and, on 
the other, that Dielasma and its palaeozoic allies and affines, when mature,were 
provided with the unpaired coiled arm of Terebratella. This assumption, in 
the first place, totally destroys the inference above made as to the primitive 
relation of the rhynchonellids to the terebratuloids and spire-bearers; and, 
secondly, would seem to necessitate a novel and unexpected interpretation of 
the brachial structure in all the spire-bearers. If Dielasma possessed the median 
arm, supported at its base by the transverse band of the loop, which corre¬ 
sponds to the jugum of the spire-bearers, then in the DiELASMA-stage of Zygo- 
SPiRA and other spiriferous shells, where this stage was well defined, there must 
also have been a median coiled arm of some extent. This median arm, in 
living forms, is due, as shown by Beecher, to the necessity of finding room for 
the cilia or tentacles multiplying at the extremities of the brachia. The mere 
presence of the transverse band in Dielasma and the DiELASMA-stage of Zygo- 
SPiRA, implies a similar extension of the brachia, and from the analogy, a median 
arm. The subsequent growth of the brachia in Zygospira, carrying the calca¬ 
reous ribbon forward, beyond the bases of the loop and into lateral spiral cones, 
would not of itself afford sufficient reason for assuming that the growth of the 
brachia at their extremities, which produced the median arm, was necessarily 
