32 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
This type of cranidium is quite often seén in collections from the 
Rochester shales, but one also sees another type, the one which Hall 
figured from the Clinton. The cranidium is similar to the one just 
described, but the glabellar furrows, instead of being short and 
straight, are long, curve backward, and their inner ends almost meet. 
This type of head deserves to be recognized as distinct from the other; 
it is the type of head figured by Hall from Wisconsin and though both 
types are very common there, this is by far the more abundant. To 
this same type, though possibly not to the same species, belongs the 
cephalon of Ceraurus bimucronatus figured by Roemer. 
No entire specimen of Cheirurus has, so far as I know, been found 
in America, and it is therefore difficult to decide what pygidium shall 
be associated with each type-of cephalon. It would appear that no 
Cheirurus pygidium had been figured from New York. The M. C. Z. 
possesses a single small pygidium of a Cheirurus from the Rochester 
shale at Rochester, N. Y. It is of the familiar Chetrurus insignis type, 
with three pairs of long slender spines, and a short median spine. It 
is very different from the pygidium from the Waldron shale ascribed 
to Ceraurus niagarensis by Hall, for that specimen was described as 
having broad flat spines, each spine with a depressed line on the sur- 
face. 
Pygidia found at Wauwatosa are like the one from Rochester and 
it seems probable that this type of pygidium is to be referred to 
Cheirurus niagarensis. 
The following description of Cheirurus niagarensis is based on three 
glabellas (M. C. Z. 325) and a pygidium (M. C. Z. 324) from the 
Rochester shale at Rochester, N. Y., and a cranidium with three 
segments attached and an associated hypostoma, from Wauwatosa, 
Wise. (M. C. Z. 626). A large cranidium with a part of the thorax 
(M. C. Z. 627) from Wauwatosa was also consulted. 
CHEIRURUS NIAGARENSIS (Hall) restricted. 
A Cheirurus of medium size. Cranidium semicircular in outline, 
gently convex, the glabella forming the highest and most prominent 
part, but not standing much above the cheeks. The glabella reaches 
the front of the cranidium, expands toward the front, and is widest 
at the middle of the frontal lobe. Dorsal furrows narrow and sharp, 
but not very deep. Glabellar furrows short, sharp, the first two pairs 
extending only a short distance onto the glabella. Their direction is 
