1092 NEW YORK STATE MUSEUM 



Cephalothorax. The cephalothorax is semielliptic or subtri- 

 angular in outline, the length equaling the breadth at the base, 

 and comprising about one fifth the length of the entire body. 

 Along the gently curving sides and acutely rounded front is a 

 narrow flattened border, striated on the under surface, and not 

 exceeding a fraction of a millimeter in breadth. The dorsal sur- 

 face is slightly rounded or arched from the side to the center, so 

 that in an undistorted shield 22 mm long, the elevation is about 

 2 mm. The posterior edge, except for a slight forward curve at 

 the genal angles, is straight. The compound eyes barely break 

 the outline of the shield ; they are small, elongate, widest anterior 

 to the middle, the outer side arcuate, the inner formed by three 

 nearly straight edges — a short basal and a little longer anterior, 

 forming slightly rounded obtuse angles with a long inner side. 

 The anterior end of the eye is acute. The length of these organs 

 on the cephalothorax, the dimensions of which were given 

 above, is 5 mm. However, the usual proportion between the 

 length of the shield and eye is as 1 :4.5. A line drawn connect- 

 ing the posterior ends of the eyes passes through the center of 

 the shield. The ocelli are situated on a small tumescence cut 

 by this line; they form two minute, ringlike prominences sepa- 

 rated by about the length of their diameter. 



Abdomen. The abdomen, at the widest point, or between the 

 third and fourth dorsal segments, is a little wider than the base 

 of the cephalothorax. Thus, in one animal measured, these dimen- 

 sions were respectively 24 mm and 17.5 mm, in a second 23 mm 

 and 17 mm, and in a third 33 mm and 26 mm, probably varying 

 somewhat according to the amount of compression and also 

 somewhat with the animal. From this point the abdomen 

 tapers very gradually to the telson; it is divided into anterior 

 and posterior parts, easily distinguished by their structure. 

 The preabdomen consists of six dorsal and five ventral, trans- 

 verse plates ; the postabdomen of six annulate segments and one 

 spiniform. 



Preabdomen. The first tergal plate of the preabdomen is very 

 narrow and is overlapped by the posterior margin of the shield. 

 Its posterior edge is slightly convex, and its ends are rounded. 



