331 



occurs in other species of this genus. In other specimens the striae are 

 more distinct and distant, but still are of the same character. In a 

 specimen hi Dr. Grant's cabinet, the middle portion of the front of the 

 head is nearly smooth, and in addition to the striae, is coarsely punctured. 

 The following are the dimensions of the original specimen (A) , and the 

 one represented by fig. 316, d, (B), in lines. 



A. B. 



Lines. Lines. 

 Length of the head in a straight line from the anterior 



to the posterior margin, 12 10 



Following the curvature, 16 14 



Distance between the dorsal furrows, 7^ 6| 



Width of head at the cheek angles, 18 15 



Distance of the eye from the dorsal farrows, 4 3 



Length of the eye, 2 2 



Distance of the eye from the posterior margin, 1 1 



Length of thorax, 9 1\ 



Width of axis at first segment, 7^ 6^ 



" " at last segment, 6 5 



Bend of the fifth segment from side of axis, 2| 2 J 



Length of the pygidium in a straight line, 1\ 7i 



Following the curve, 9 9 



Greatest width, 14 12J 



This species is alhed to the /. crassicauda (Wahlenberg), but still is, 

 I think, quite distinct. In the two specimens of the latter, figured by 

 Dalman, the axis in one is 9 lines in length, and Si in width ; in the 

 others, it is 10 lines in length, and 7i in width. In the specimen figured 

 by Angelin, it is 1Q\ lines in length, and 7i in width. The axis appears 

 thus to be proportionally longer and narrower than it is in /. Americanus. 

 The pygidium is also larger in I. crassicauda, and the surface of the head 

 marked by coarse undulating lines, not short wrinkle-shaped fissures. 



From /. consimilis (ante, p. 277), this species diflFers in the form of 

 the glabella, and in the surface characters, but more remarkably in the 

 shape of the posterior angles of the head, as shown in the following 

 fisrures : 



Fig. 317. Fig. 318. 



Fig. 317. — A detached cheek of 7. consimilis. 

 " 318. — Side view of a head of I. .Americanus. The dotted line, a, is au outline of 

 the cheek of /. consimilis. 



