279 



furrows, and distant from the posterior margin about half the width of the 

 central lobe. They are not conical, but depressed hemispherical, abruptly 

 elevated on the outside, and on the inside even with the surface. 



Thorax with ten segments ; axis well defined, convex, slightly tapering 

 backwards, with straight sides, about one-third the whole width ; side 

 lobes flat to the angle of the pleurae, which is situated at about half the 

 width of the lobes. 



Pygidium sub-semicircular, its length a little greater than half the 

 width ; anterior angles obliquely truncated, all the rest of the posterior 

 margin uniformly rounded ; axis convex, conical, well defined at the sides, 

 but not at the apex, about one-third the length. 



Surface having a smooth aspect, but when closely examined presenting 

 the usual concentric fissure-like markings. 



With one exception, all of the specimens of this species that I liave are 

 small, being mostly the heads and tails of individuals which, when perfect, 

 were about one inch in length. There are in the collection three small 

 perfect ones, rolled up, about 4 lines wide. One imperfect pygidium is 

 9 lines in length, and probably belonged to an individual IJ inches in 

 length. 



This species differs from I. fratemus, in having the eyes nearer 

 the dorsal furrows, and more distant from the posterior margin, and also 

 in the great convexity of the front part of the head. 



It is, like the last, most probably a geographical variety of /. crassi- 

 cauda. 



Locality and Formation. — P, Cow Head, Newfoundland ; Quebec 

 group. 



Collector. — J. Richardson. 



Ill.s;nus arcuatus. (N. sp.) 



Fig. 265. 



Description. — Head very short, curved into the shape of a crescent, 

 the angles produced backwards into stout spines, which terminate acutely. 

 Glabella quadrate, extending a little more than half the length of the 

 head, measured along the surface. The posterior half of the head is 

 somewhat flat or gently convex ; the anterior half is abruptly curved 

 down, and the margin a little inrollcd or brought under tlie most pro- 

 jecting part of the front. Eyes small, depressed hemispherical, distant 

 from the dorsal furrows the width of the glabella, and half that distance 

 from the posterior margin. 



