BRACHIOPODA. 343 



In the study of the multifold variations of the articulates bearing calcified 

 spiral brachial supports, the Helicopegmata 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 

 spiniforra 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 laminaa 

 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 tho.se 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 Rhtncbonbllidm. affects only the crura, and in the 

 Stropbombnid^, 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 



