912 Report or THE State GEOLOGIST. 
hinge line and no cardinal area. The pedicle-opening is a broad 
triangular fissure, the valves being in contact behind only at and 
near the cardinal angles. Though this shell is from the Lower 
Cambrian faunas there are other brachiopods of obolelloid type 
that are quite as ancient; still Parmrmna exemplifies the source 
from which the development of more complicated forms has 
proceeded. 
From this simplest of all known (and it might be fair to say, 
all possible) brachiopods, the development and specialization of 
generic characters proceeded in various directions. The orbicu- 
lar form and preponderating chitinous shell-substance form 
essential traits of a considerable group of primeval genera repre- 
sented by Oxo.tus, OpoteLia, ELxnta, etc., in which, however, 
decided progress is evinced in the rapid specialization of the mus- 
cular apparatus as well as in the restriction of the pedicle-passage 
to a narrow slit. Some closely allied forms with highly chitinous 
shell and obolelloid * muscular scars present an elongate rather 
than subcircular outline, and in this respect indicate a departure 
toward the characteristic linguloid exterior. This is first shown 
by the Cambrian and early Silurian genera LinevuLetia, Linev- 
Lops and Lrpropo.ts. 
The true Lryevta, with its highly complicated muscular 
apparatus, appears to have become established and static some 
time after the opening of the Silurian (Trenton fauna) and we 
have no reason for believing that the type of structure fixed at 
that remote period differs essentially from the typical Lingulas 
of existing seas. It is not that offshoots and modifications were 
not given off, especially during the early periods of its exist- 
ence, but such lateral departures were short-lived and the old 
firmly established generic type, resistant to variation in physical 
surroundings or adapted to the whole vicissitude of marine con- 
ditions, has perpetuated itself without modification as far as 
indicated by the structure of the shell. 
GuosstnA, Dianomta, Barroisetta and Tomasrna, which repre- 
sent early deviations from Lineurta during the Silurian and 
Devonian periods, embody no substantial variations, though the 
last two demonstratea gradual assumption of articulating processes. 
* OBOLELLA is a more elementary type than Ozo.us, as is strongly evinced by the character 
of the muscular scars, In Ogouus the latter are complicated and approach those of Lineua. 
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