THE NATURAL HISTORY SURVEY. 221 



than the^length. Dorsal furrows well defined to points opposite 

 the anterior extremities of the eyes, from there describing a 

 gently sigmoidal curve and becoming much fainter or some- 

 times even obsolete, terminating in slight pits which are situa- 

 ted a little nearer the anterior margin than the anterior ex- 

 tremities of the eyes. Eye-lobes depressed below the median 

 portion of th,e cephalon, convex, sloping laterally. Eyes ob- 

 lique, describing less than ' a semi-circle, situated posteriorly, 

 three or four times as long as high. Posterior limb of the facial 

 suture directed obliquely outward, cutting the posterior mar- 

 gin about two-fifths the distance from the posterior extremity 

 of the dorsal furrow to the genal angle; anterior limb twice as 

 long as the posterior, describing a gentle outwardly convex 

 curve, cutting the anterior margin nearly in front of the anterior 

 extremities of the eyes. Free cheeks sub-triangular, longer 

 than wide, depressed in a rounded furrow around the outer 

 margin -of the eyes and then gently convex to the lateral and 

 posterior margins. Genal angles rounded. 



The pygidium is depressed convex becoming slightly con- 

 cave near the antero-lateral angles, much wider than long, the 

 antero-lateral angles obtuse; posterior margin broadly and 

 regularly rounded with a broad doublure; the anterior margin 

 arcuate, slighly sinuate or notched on each side about half 

 way between the center and the antero-lateral angles. Just 

 within the anterior margin between the axial notches is a shal- 

 low rounded marginal furrow which becomes deeper and stronger 

 laterally beyond the axial notches and then becomes obsolete 

 before reaching the lateral margins. A slight median rib is 

 usually present upon the surface, extending from the posterior 

 margin towards the front, becoming obsolete before reaching 

 the anterior margin. 



The dimensions of a nearly perfect cephalon .are: length 

 20.5 mm., width 39 mm., convexity 23 mm., width between 

 the extremities of the eye-lobes 3 1 mm. 



Remarks. This species has usually been confused with 

 /. ioxus, but it differs from that species in its smaller size, the 

 greater convexity of its cephalon, the longer dorsal furrows, 

 in the more regularly rounded anterior margin of the cephalon 

 and in the median rib of the pygidium. The species much more 

 closely resembles I. armatus, differing from that species chiefly 

 in the absence of the short genal spines and in its greater con- 

 vexity. In general form the cephalon resembles that of /. 

 harrisi, but the eyes are less protuberant, the dorsal furrows 



