AUG. F. FOERSTE 
The exact correlation of strata by means of fossils requires an 
equally exact discrimination of these fossils into species and 
varieties. Especially is this true of genera in which many of 
the species are closely related. In the genus Calymene^ for in- 
stance, almost any Ordovician species has been called C. senaria, 
while almost any Niagaran species has been called C. niagarensis. 
Recently Raymond (Bull. Mus. Comp. Zoology, Harvard Univ., 
60, 1, 1916, p. 27) has placed the Niagaran species on a better 
footing. In a similar manner, the various species in the Cin- 
cinnatian strata of Ohio, Indiana, and Kentucky need more 
exact discrimination. Very little has been published on the 
species of Isotelus in the Cincinnatian strata. Moreover, the 
published figure of Encrinurus ornatus was altogether insufficient 
for purposes of discrimination from closely similar forms found 
in other strata. The following pages are intended as a contribu- 
tion to a more exact discrimination of some of the closely 
similar forms of the genera mentioned above. 
Isotelus brachycephalus sp. nov. 
Plates XIV, XIVA, and XV 
Cephalon (plate XIV) 10.5 cm. in length along its median 
line, and 26.3 cm. in width at the genal angles; the ratio of the 
length to the width being four-tenths. The marginal border is 
depressed or concave for a width varying from 13 mm. anteriorly 
to nearly 20 mm. along the free cheeks. The facial suture is 
outlined distinctly both anteriorly and posteriorly to the pal- 
pebral lobes, and the position of these lobes is indicated, but the 
course of the suture along the margin of the palpebral lobes can 
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