SPIRIFERiE OF THE CHEMUNG GROUP. 
941 
becoming quite flat in the more extended forms : mesial fold usually 
strongly defined, rising abruptly above the surface on either side, and 
marked along the centre by a deep groove. The area is narrow-linear. 
Surface marked by from ten to fourteen rounded or subangular plications, 
those on either side* of the mesial fold and sinus bejng usually stronger 
than the others. When the shell is fully preserved, and sometimes in 
partial casts, the surface is marked by strongly arching lamellose strise 
or lines of growth, which become much crowded towards the margin 
of the valves. 
Little is known of the interior structure of this species. A cast of the 
ventral valve shows extended and not greasly divergent dental lamellse, 
with a strong longitudinal median crest or septum in the muscular im¬ 
pression. 
The specimens under examination are nearly all in the condition of partial casts 
or with the shell more or less exfoliated; while all are in single valves, adhering 
to the sandstone or shale in which they are imbedded. 
The species is well marked and quite distinct from any other in this formation, 
though showing a wider range of variation in forms and proportions than I had 
originally supposed. The short forms with extremely extended and mucronate car¬ 
dinal extremities, which are figured in the Report of the Fourth District, p. 270, 
f. 5 & 5 a, prove, by several gradations of form, to be of the same species. This 
variety resembles the Spirifera mucronata , for which it has no doubt sometimes 
been mistaken. 
Among specimens from Ithaca (N.York), there are numerous small individuals 
of this species, which, on a cursory examination, might be regarded as the young 
of S. mucronata. The specimens are for the most part casts, and the dorsal valve 
shows the duplication of the mesial fold; but there is no (or scarcely any) in¬ 
dication of the plication in the bottom of the sinus, although there is unmistaka¬ 
ble evidence of a longitudinal septum extending from the beak to near the base 
of the muscular impression, a feature not known to exist in S. mucronata. The 
specimens are usually semielliptical and little extended on the hinge-line, but in 
some individuals there is a great extension of the cardinal extremities. From 
.these small individuals there is an almost insensible gradation to the larger forms, 
as shown in the illustrations on Plate xl. In the laterally extended forms the area 
is low, while in the shorter and more rotund forms it is more elevated, as shown 
in the figures. The more gibbous forms have been found in a compact sandstone on 
the Genesee river, while the smaller and attenuated ones are from more easterly 
[ Paleontology IV.] 31 
