346 PALEONTOLOGY OF NEW YORK. 



SPIRELLA, of the St. Cassian beds, are double-spiraled athyroids ; Kayseria, of 

 the middle Devonian, which is the only double-spiraled form known in the 

 Palaeozoic, appears to be an aberrant and accelerated representative of the 

 stock which by more gradual development produced Retzia and Eumetria. 



Only one large group of spire-bearing shells retains the cardinal area, namely, 

 the Spirifkridm, a family with everted spirals, one of the earliest to appear and 

 the la't to disappear. Its abundant representatives possess the longest of 

 spirals, and for the most part these are greatly extended transversely, held at 

 arm's length as it were, unsupported by a connecting jugum (except in the 

 sparsely represented genera Cyrtina and Spikiferina) ; but in spite of the deli- 

 cacy of the structure and its apparent mechanical disadvantages in the absence 

 of a continuous jugum, this type of structure has maintained its distinctive char- 

 acter and multiplied in a most remarkable manner. 



The relations of the brachiopods with spiral brachidia to the Ancylobrachia, 

 or those shells commonly spoken of as the terebratuloids, has been a fruitful sub- 

 ject of discussion, and given rise to investigationsof great astuteness and merit. 

 Reference has already been made to the facts established by Beecher and 

 ScHUCHERT, from the development of the brachidium in Zygospira, which show 

 that this atrypid passes through a growth-stage in which the brachidium has a 

 simple terebratuloid form, similar to that in the mature condition of Dielas.ma; 

 that the spirals are formed by the continued growth of the descending lamellae 

 of the loop beyond the point of their recurvature into the ascending lamellae. 

 What is thus true of Zygospira we must assume to have been equally true of 

 all the spire-bearers, and the analogies thus established between them and 

 the loop-bearing shells are these : The entire loop in Dielasma, Cryptonella, 

 etc., corresponds to that portion of the brachidium, in the spire-bearing forms, 

 which lies behind the anterior basal edges of the jugum; the descending lam- 

 ellae of the former represent only the posterior portion of the primary lamellae 

 of the latter, while the ascending lamellae and transverse connecting band of 

 the Ancylobrachia are the equivalent of the jugum in the spire-bearers. The 

 spirals, however, are a later development in the individual, and are hence 

 undoubtedly a subsequent phyletic condition. Hence it is Inferred that the 



