415 



Just behind the margin there is a single groove, obscure towards the 

 axis but more distinct outwards. There are in some specimens, several 

 faintly marked ribs, but in general the side lobes, with the exception of the 

 anterior furrow, are smooth. 



Length of head of a specimen of medium size, seven lines width, twelve 

 lines ; distance between centre of eyes, seven lines. 



Length of a pygidium of medium size, six lines ; width, twelve lines ; 

 length of axis, four lines and a-half ; width of same at front margin, three 

 lines and a half, and at half a line from the posterior extremity, three 

 lines. 



I have seen the underside of the head of this species and the sub-rostral 

 fold is distinctly divided as in A. platyeephalus. Had not this character 

 been observed, I would have, without much hesitation, referred the head 

 to the genus Illcenus. 



In Limestone, No. 3. 



ASAPHUS aONIUEUS. (N. sp.) 



The above name is proposed for a small triangular pygidium found in 

 No. 3. It is evidently distinct from any described Silurian species of 

 this country, but allied to one that occurs in the Chazy Limestone at Min- 

 gan. The form is triangular, the length three-fifths, or thereabouts, of the 

 width, the axis scarcely at all elevated above the surface, and indistinctly 

 divided into segments in the anterior half, but towards the extremity 

 becoming strongly elevated, smooth and pointed. The largest specimen 

 seen is about half an inch in length. It resembles the tail of a small 

 Sbmalonotus. 



Limestone, No. 3. 



