Braoxroropa. ; 929 
derivation. This latter question must long be a matter of legiti- 
mate speculation, and in view of this fact few arguments of such 
a nature in this place will be permissible. The living representa- 
tives of RayncHoneLia and Trresratuta are animals in which a 
very considerable part of the brachia does not become sufficiently 
spiculized to form a continuous calcareous support. In RP. (Hemi- 
thyris) 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 lamellz known as the crural processes. All of the 
living AnorLopracuia which possess a long recurved loop like 
that of Cryproyeria and Drierasma of the Paleozoic, have an 
unsupported median unpaired spiral arm, coiled in 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 the 
fossil rhynchonellids and terebratuloids from the adult condition 
of the brachia in their nearest living representatives, it becomes 
necessary to assume that on the one hand the paleozoic rhyn- 
chonellids possessed long coiled spiral arms, and, on the other, 
that Dimiasma and its palzozoic allies and affines, when mature, 
were provided with the unpaired coiled arm of TrrEBraTELa. 
This assumption, in the first place, totally destroys the inference 
above made as to the primitive relation of the rhynchonellids to 
the Ancytopracaia and Hericopremata; and secondly, would 
seem to necessitate a novel and unexpected interpretation of the 
brachial structure in all the spire-bearers. If Drmxasma possessed 
the median arm, supported at its base by the transverse band of 
the loop, which corresponds to the jugum or the spire-bearers, 
then in the Drmtasma-stage of Zyeosrrra 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 Brrcusr, to the necessity of 
finding room for the cilia or tentacles multiplying at the extremi- 
ties of the brachia. The mere presence of the transverse band in 
Dretasma and the Distasma-stage of Zyaospira implies a similar 
, extension of the brachia, and from the analogy, a median arm. 
The subsequent growth of the brachia in Zycosrrra, carrying the 
calcareous ribbon forward, beyond the bases of the loop and into 
lateral spiral cones, would not of itself afford sufficient rea- 
117 181 
