PAPERS ON GEOLOGY 271 
ARTHROPODA 
TRILOBITA 
Isotelus brevicaudatus n. sp. 
Plate I, Fig. 6, 7 and 8 
Description: Cephalon transversely semi-elliptical in out- 
line, length along the median line about 22 mm. which is a little 
less than half the width, genal angles rounded; occipital groove 
well defined. Glabella not distinctly defined, moderately con- 
vex in the median portion, slightly angular at its anterior ex- 
tremity, width between the eyes slightly more than 12.5 mm., 
or a little more than half the greatest length of cephalon; 
eyes situated at or slightly posterior to the mid length of the 
glabella. Greatest width of glabella anterior to the eyes 23.5 
mm. The facial sutures anterior to the eyes curve outward 
rather strongly to near the margin whence they continue around 
the anterior portion of the cephalon. Posterior to the eyes the 
facial sutures are directed backward and outward, cutting the 
posterior border of the cephalon more than half the distance 
between the points directly behind the inner margins of the 
eyes and the genal angles. Free cheeks about 9 mm. broad 
at the widest part, nearly twice as long as wide, the widest 
part opposite the posterior border of the eyes, the outer mar- 
gin gently curved or nearly straight, and the genal angles 
rounded. Thorax not known. 
Pygidium transversely subelliptical, a little longer than the 
cephalon, length 26.5 mm., width a little less than twice the 
length, ratio of length to width as 7 to 11; axis poorly de- 
fined, not extending to the posterior extremity, no segments 
visible on the axis or on the pleura, anterior border convex, 
antero-lateral margin rounded, posterior border broadly round- 
ed. Marginal border slightly exfoliated, 6 mm, wide, and 
marked by fine lines sub-parallel with the lateral margins. 
Fragments of the test of this species are very numerous in 
« Narrow zone in the basal part of the Thebes sandstone where 
they are associated with Endymtonia bellatula. 
The species above described somewhat resembles Jsotelus 
susae Whitefield in general outline of cephalon and pygidium, 
but differs from that form in the much greater constriction of 
