THE MODIFICATION OF GENERIC CHARACTERS. 



markable degree of expansion of the main features of the 

 variable characters appears very early in the history of each 

 genus. 



As an illustration of the rapid appearance of the full 

 quota of extrinsic modifications of a new intrinsic element of 

 structure we may examine the history of the spiral brachial 

 appendages in the suborder Helicopegmata. 



Statistics of the Life-history of the Spire-bearing Brachiopods 

 (Helicopegmata). The earliest trace of the spire-bearing 

 Brachiopods is in the Ordovician, in a single simple form, the 

 genus Zygospira. 



At the next faunal stage, the base of the Upper Silurian 

 system, there were representatives of each of the families into 

 which the known Helicopegmata are divided (Atrypidae, 

 Spiriferidae, and Athyridae); and of the twelve subfamilies 

 into which the seventy-three recognized genera are distributed, 

 nine are also known from as early a stage as the Upper Silu- 

 rian (viz., Zygospirinae, Dayinae, Atrypinae, Suessiinae, Tri- 

 gonotretinae, Rhynchospirinae, Hindellinae, Athyrinae, and 

 Meristellinae). Of the others, Uncitinae, first appearing in 

 the Devonian, has the same kind of brachidium as the sub- 

 family Suessiinae; and the loop of Diplospirinae, appearing 

 first in Kayseria of the Devonian and having several genera 

 in the Triassic, is rather to be considered as an extreme differ- 

 entiation of the Athyroid type; and so far as the brachidium 

 is concerned, Koninckinina of the Mesozoic is also an extreme 

 differentiation of the same Paleozoic type.* 



The Rapid Appearance of the Different Modifications of the 

 Brachidium. For the present discussion it matters not whether 

 the calcification of the spirally-terminated brachidium of the 

 Helicopegmata is a modification of that seen in the loop of 

 the Ancylobrachia, or whether it arose from a form in which 

 there was no calcified support ; for both of the suborders, so 

 far as evidence is at hand to show, first appeared in the Or- 

 dovician. 



One intrinsic character distinguishing these suborders from 

 all the previously existing Brachiopods is found in the presence 



*In this discussion I have followed Schuchert's "A Revised Classification of 

 the Spire-bearing Brachiopoda," Am, Geol , vol. xm. p. 102, etc., Feb. 1894. 



