NORTH AMERICAN SPECIES OF THE GENUS ATAX 207 



inward and forward about half way acrosss the plate formed by the two. 

 This plate is nearly rectangular, somewhat longer than broad, with the 

 anterior margin slightly concave, and the inner slightly convex. On the 

 thickened border at the inner posterior angle are two hairs. 



The legs, as before stated, are very long, all considerably exceeding 

 the body in length. Measurement of several specimens show a variation 

 in the relative length of legs and body and in the relative lengths of the 

 legs themselves, but this variation seems to possess neither locality, sexual 

 nor specific significance. In a single specimen from Lake St. Clair the 

 third pair of legs exceeds in length the first, but in all other cases III is 

 the shortest, nearly equalling I, and II and IV are considerably longer, 

 with IV a little longer than II and more than twice the length of the body. 

 In all cases the order of length of the individual segments in II and III is 

 as follows, beginning with the longest — 5, 4, 6, 3, 2, 1; in I, 5 and 6 are of 

 about equal length, and both shortened, being shorter than 3; in IV, 8 is 

 lengthened and surpasses 4. In the case of I, 2 possesses a ventral 

 papilla, projecting chimney-like to a distance equal to two-thirds of the 

 thickness of the segment, deeply excavated and open at one side and re- 

 ceiving into this excavation a long movable spine, while there is also a 

 short movable spine at the outer margin of the posterior side; 3 has two 

 movable spines on either side ventrally; 4, four in two ranks on the 

 proximal half of the ventral surface; 5 a row of four towards the base 

 and one at the tip on the ventral side, of the former the first and third 

 being the longer. All of these spines are widest at a little distance from 

 their insertion tapering gradually to a blunt tip, and more rapidly to 

 their point of insertion. The fifth segment has the dorsal surface pro- 

 duced forming a scoop-like shield over the insertion of the next segment. 

 1, 2 and 3 are very stout, 4 tapers considerably towards the tip, 5 and 6 

 being noticably slenderer and 6 somewhat curved. II, III and IV are 

 slenderer at the base than I, and taper gradually from base to tip; they 

 possess a moderate number of long, slender, straight spines which form 

 groups on the tips of segments II 5, III 4, III 5, IV 4 and IV 5, while on 

 IV 3, numerous spines are scattered along the ventral surface. On I the 

 claws are rather stout, much curved and bifid, the ventral of the two tips 

 being the stouter; on the other legs, however, the claws are longer, much 

 more slender and the two tips are long, slender and shai'ply pointed, 

 while the dorsal of the two is not so long as the other and thus forms an 

 accessory claw arising at a distance from the tip equal to one-fourth the 

 length of the claw. 



The genital cleft in the female is flanked by four chitinous plates, of 

 which the two anterior are irregularly rectangular with rounded angles 

 and the two posterior roughly triangular with the angles also rounded. 

 Each bears three acetabula and numerous slender spines, and six longer 

 and stouter spines are borne — two each by the anterior plates and one 

 each by the posterior — on their contiguous angles which are somewhat 

 produced and directed outward. The male also possesses twelve 

 acetabula but they are borne on two lunate plates, one on either side, 

 and are distributed on each plate in two groups of three each. 



