328 



276), and has given me much perplexity on that account. The differ- 

 ences are that in this species the eye is proportionally nearer the glabella, 

 and the glabella itself wider in proportion to the length of the head 

 measured along the curve. Four specimens, a, b, c, d, gave the following 

 measurements, in lines : 



A 

 Length of the head on the curve . . . . 24 



Width of the glabella 13 



Distance of the eye from the dorsal furrow 3i 



From the above, it appears that the width of the glabella is about half 

 the length of the head, and the distance of the eye from the dorsal furrow 

 is a little less than one-third of the width of the glabella. In /. fraternus 

 the eye is distant from the dorsal furrow at least one-half the width of the 

 glabella, while the latter is less than half the length, of the head. A 

 specimen, of which the length of the head is 8 lines, has the glabella 3| 

 lines in width. In this proportion an individual with the head 24 lines in 

 length would have the glabella 8i lines in width, or nearly 4| narrower 

 than the specimen of I. simulator, lettered a above. 



A number of other specimens of the head of I. fraternus have similar 

 proportions ; none of them are more than 10 lines in length, and most of 

 them have a small tubercle near the posterior margin on the glabella. 

 I. fraternus appears thus to be a smaller species with a narrower axis, 

 and the eye more distant. 



I. Crasdcauda has likewise a more slender axis, and the pygidium 

 more broadly rounded. 



I. Davisii (Salter) has the doublure on the under side of the pygidium 

 less than one-third the length, Avhile in this species it is more than half 

 the length of the pygidium. The anterior angles arc not so deeply trun- 

 cated as they are in our species. 



Locality and Formation. — Stanbridge, range 6, lot 20 ; Quebec group. 



Collector.—']^. C. Weston. 



Ill^nus consimilis and I. Americanus. 



In comparing these two species with each other, it was stated (^ante, p. 

 278,) that the only differences were in the form of the middle lobe of the 

 head, and in the surface markings. Having recently obtained a number 

 of specimens of /. Americanus, and also some additional fragments of the 

 head of /. consimilis, I am now enabled to shew that these are really 

 quite distinct species. 



