346 
PALEONTOLOGY OF NLW YORK. 
SPIRELLA, of the St. Cassian beds, are double-spiraled atbyroids; Kayseria, of 
the middle Devonian, wbicb 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 Spirifeiud^, 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 jugurn (except in the 
sparsely represented genera Cyrtina and Spiriferina) ; 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 investigations of great astuteness and merit. 
Reference has already been made to the facts established by Beecher and 
ScHUCHERT, from the development of the brachidium in Zygosfira, 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 Zvgospira 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 Ancylobrachta 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 
