918 Report oF THE State GEOLOGIST. 
has been shown that Autorgina cingulata may retain a pedicle- 
covering or external sheath, in fact a true deltidium bearing an 
apical perforation, like that in Ctirampontres. The same charac- 
ter is highly developed or fully retained at maturity in Ipurpga. 
This is evidence of the highest moment, and conclusively shows 
the line along which the clitambonitoids and strophomenoids have 
been derived. It is an immediate departure from the primitive 
type of the brachiopod into the articulate subtype. 
Passage from the inarticulate to the articulate plan of structure 
was thus effected at a very early period; indeed, almost at the 
outset of the history of the group. The continuance of the two 
types has since been that of diverging series, constantly widening 
the structural gap between them. We have no irrefragible evi- 
dence that this chasm has been bridged at any other point than 
near its sourcé; the inclinations from the one type toward the 
other, shown in the articulating processes of BarrotsetLa, Tom- 
ASINA, etc., represent uncompleted accessory lines of development, 
which were abruptly terminated without accomplishing the full 
transitions. Such forms have left no descendants, so far as 
known. 
The most elementary structure, then, observable, among the 
_ Articulate Brachiopods is the combination of the deltidium with 
a distinct pedicle-cavity, whose anterior margins are not free, 
and whose lateral walls or dental lamelle are not highly 
developed; these features being accompanied by gently and 
unequally biconvex valves, well-defined cardinal areas and 
elongate hinge-line; producing, in effect, a generally orthoid | 
expression both of interior and exterior. This is the condition 
of Brixrinesetta of the Cambrian, Orthis loricula and O. deflecta 
of the Trenton group, and 0? laurentina of the Hudson River 
fauna, and it is continued without essential modification, except 
in the gradual contraction of the pedicle-cavity and deltidium, 
into Srropnomena of the Silurian, its allies and successors, 
OrrnorHetss of the Devonian, and Drrsya of the Carboniferous, 
Hiprarionyx, Trrececia, StREPTORHYNcHtS, etc., into Lepra ana, 
Rarivesquina, SrropHEoponta, PrecramBoniTes, Cuongtes and 
Propvctvs. 
The tendency to contract the pedicle-cavity and deltidium 
presents its extreme manifestation in the Devonian forms of 
170 
