BracuHiopop. 927 
beds, are double-spiraled athyroids; Kaysrr1a, of the middle 
Devonian, which is the only double-spiraled form known in the 
Paleozoic, appears to be an aberrant and accelerated representa- 
tive of the stock which by more gradual development produced 
Rerzia and Eumerrta. 
Only one large group of spire-bearing shells retains the cardinal 
area, namely, the Sprrirerip®, a family with everted spirals, one 
of the earliest to appear and the last 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 
more spaisely represented genera Cyrtimva and Spreirerina), but 
in spite of the delicacy of the structure, its apparent mechanical 
disadvantages in the absence of a continuous jugum, this type of 
structure has maintained its distinctive character and multiplied 
in a most remarkable manner. 
The relations of the brachiopods with spiral brachidia, to 
the AncyLopracsts, or those shells commonly spoken of as the 
terebratuloids, has been a fruitful subject of discussion and 
given rise to investigations of great astuteness and merit. 
Reference has already been made to the facts established by 
BrecueEr and Souvucuert from the development of the brachidium 
in Zyeosrira, 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 Dretasma; that 
the spirals are formed by the continued growth of the descending 
lamelle of the loop beyond the point of their recurvature into 
the ascending lamellz. What is thus true of Zyeosrrra we must 
assume to have been true of the Hxricopremara generally, and 
the analogies thus established between them and the loop-bearing 
shells are these ;— the entire loop in Dretasma, Cryrronetra, etc., 
corresponds to that portion of the brachidium, in the spire-bear- 
ing forms, which lies behind the anterior basal edges of the jugum; 
the descending lamellz of the former represent only the posterior 
portion of the primary lamelle of the latter, while the ascending 
lamellae and transverse connecting band of the AnoyLopRracHia 
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 
179 
