CRUSTACEA. 
29 
The outline of this cephalon is nearly crescentic, its length one-half its width, 
and the genal angles produced into conspicuous spines. The glabella agrees 
with that of D. Hausmanni, both in proportions and character of lobation. Its 
frontal lobe bears three circular depressions, one on the axial line and one near 
each of the lateral extremities; of these the former is largest and deepest. 
The eyes are large and elevated, the orbital sulcus unusually broad and con¬ 
spicuous ; the cheeks slope abruptly in front, and more evenly toward the genal 
angles, and their surfixce is marked by irregular, co.nspicuous wrinkles, except 
where it is crossed by the broad, smooth sulcus in which the facial suture lies. 
The border is a very characteristic feature, being moderately broad, smooth and 
concave on the sides, narrowing near the extremities of the frontal glabellar 
lobe and produced in front into a sub-nasute extension. Just below the lateral 
extremities of the glabella begins a series of marginal crenulations or alternate 
thickenings and thinnings, which become more conspicuous anteriorly. 
The border thus has a character similar to that of D. anchiops, and is 
of especial interest as suggesting the inception of that peculiar frontal orna¬ 
mentation which is more fully developed in Odonfocepholus. There is no doubt 
that this is the cephalon of D. pleuroptyx, as in one well-defined example the 
parts have been found in conjunction. The cepliala which were referred to 
this species on plate Ixxiv of the third volume of the Palaeontology of New 
York, with the possible exception of figures 5 and 9, must therefore belong to 
another species. No similar heads have been found in the Helderberg mount¬ 
ains. The original pygidium of D. micrurus, Green, is known to have come 
from Schoharie, although the author regarded it as from Trenton Falls. At 
Schoharie the large pygidia which agree with the type of D. micrurus are not 
uncommon, and the cephala with which they are there associated, and those in 
the western extension of the Lower Helderberg formation, in Herkimer county, 
should probably be regarded as belonging to this species. The cephala 
described here as that of B. pleuroptyx appear to be of rare occurrence at 
Schoharie, and that species seems to have been most abundant in the 
Shaly Limestone, and D. micrurus in the Lower Pentamerus Limestone. 
The entire young individual figured on plate Ixxiv (Pal. N. Y., vol. iii), may 
