BRACHIOPODA. 
343 
In the study of the multifold variations of the articulates hearing calcified 
spiral brachial supports, the HELicorEGMATA of Waagen (1883), the conclusion 
has enforced itself that the degree of solidification of the brachia in this group 
is to be regarded as an index of differentiation. To illustrate; there is no evi¬ 
dence for assuming that the single volution made by the spiral in Protozyga 
and Hallina represents an incomplete spiculation of the brachia, or that the 
spiniform and discrete jugal processes in Spirifer, persisting throughout the 
genus, do not'fully exemplify the adult condition of the jugum (=loop) in 
these shells. The mode of spiculation of the brachia in such of the living 
terebratuloids, in which the solidification is direct, or without complicated met¬ 
amorphoses, is on the whole confirmatory of this inference; but as there is no 
living representative of the spire-bearing forms, evidence in regard to the mode 
and degree of spiculation in this group, derived from the existing loop¬ 
bearing shells in which the brachial supports pass through highly complicated 
metamorphoses, is not altogether germane. In such intricate structures as the 
brachidia of Athyris, Kayseria, Koninckina, etc., there can be little doubt that 
the calcified apparatus represents the full extent of the fleshy brachia simply, 
if for no other reason, because the further expansion of the brachial laminae 
would not be possible for want of space. Moreover, in the spiculation of the 
spirals in all these old shells there have been no changes of form in later growth 
except those proceeding from the normal process of resorption and deposition 
necessary for increase in size and length. The reason why the spiculation 
should be complete in the spire-bearing forms, while in the Ancylobrachia or 
the terebratuloids, it does not extend beyond the loop or the lateral extensions 
of the brachia, but in the Rjiynchonellidje affects only the crura, and in the 
SmopuoMENiDyE, does not occur, even in the most elementary condition, is for 
future investigations to ascertain. 
The form of the paired spirals varies but little except under the necessity of 
conforming to the interior cavity of the valves. Their inclination and direc¬ 
tion is a feature of much significance when considered with reference to the 
development of the entire shell. It is the loop, or to employ a term more 
appropriate in view of the homologies of the spire-bearing and loop-bearing 
