83 
lobe of the abdomen is much broader than the lateral 
lobes, and has 18 distinct, double articulations. The 
side lobes are curved, and each costal arch is flatten- 
ed anteriorly near their lower extremities, no doubt 
for the purpose of enabling the animal to roll itself 
into a ball. The tail is tapering, and is composed of 
ten articulations. The crustaceous covering is here 
more thickly deposited than on any other part. The 
entire shell seems to have been covered with minute 
elevated dots; these are beautifully distinct on the 
buckler and on the tail. Whole length of the speci- 
men described, not quite two inches. 
The other specimen of this species in the cabinet 
of the Albany Institute, is a large caudal end, three 
inches and a half long, entirely perfect. Both of 
these fossils were brought from Williamsville, Nia- 
gara county, New York. They occur in a dark shelly 
limestone, filled with other petrifactions. The cal-. 
careous matter which has mineralized the trilobite, 
in this instance, as in most others, is of a much darker 
_ hue than the surrounding rock. 
Genus CERAuURUsS. Green. 
Body, very much depressed, and slightly tapering. 
Buckler, scarcely trilobate; cheeks large, flat, with 
small remote oculiform tubercles; posterior angle of 
the buckler spinous. 
Abdomen, with twelve articulations. 
Tail, rounded at the end, but terminating on each 
side with two slightly curved spines. 
