32 bulletin: museum of comparative zoology. 



This type of cranidium is quite often seen 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 himucronatus 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 Cheirurus insigiiis type, 

 with three pairs of long slender spines, and a short median spine. It 

 is very dift'erent 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. €25) and a pygidium (M. C. Z. §24) 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 



