NORTH AMERICAN SPECIES OF THE GENUS ATAX 233 



11. ATAX YPSILOPHORUS (BONZ). 



Acarus ypsilophorus Bonz, 1783; VII, 52, PI. I, f. 1-4. 



Trombidium notatum Rathke, 1797; IV, 175, PL X. 



? Hydrachna triangularis Say, 21; II, 79: id., Leconte's Ed., 59; II, 22. 



Limnochares anodontae Pfeiffer, 24; pt. 2, 27-28, PI. I. 



Hydrachna concharum von Baer, 27; XIII, 590, PL XXIX. 



Unionicola oviformis, U. lactea, 1 U. personata, V U. humerosa, ? U. 

 symmetrica, ? U. proxima, ? U. lugubris, 1 U. unicolor, ? U. reticulata — 

 Haldeman, 42; 1-3, PL, f. 1-11. 



Non Atax ypsilophora von Beneden, 48; {=A. intermedins Koenike). 



Atax ypsilophora Garner, 64; 114: Claparede, 68; 474, PL XXXIII: 

 Bessels, 69; 146: Lebert, 79; 367. 



Atax ypsilophorus Neuman, 80; 26, PL I, f. 2: Koenike, 87a; 626: 

 Haller, 81a; 78: Koenike, 82; 265: Leidy, 83; 44: Harrington, Fletcher and 

 Tyrrell, 84; 140. 



Atax concharum Krendowsky, 85; 59. 



Atax ypsilophorus Barrois and Moniez, 87; 5: Girod, 89: XIV, n. 5, 

 107: Koenike, 91c; 257: Piersig, 94b; 214: Koenike, 95b; 217: Piersig, 96; 40: 

 Koenike, 96; 232: Piersig, 97; Lief. I, 43. PL II, f. 3, a-h. 



This, the best known of all the species of the genus and the one first 

 described, is also one of its larger representatives, the males measuring 

 from 1.1 to 1.3 mm. in length, the females from 1.3 to 1.5 mm., or when 

 distended with eggs, 1.8 mm. The body is oval, but more nearly elliptical 

 than in most species, evenly rounded at each end, and much arched. The 

 males are considerably slenderer than the females. 



The eyes are proportionately small and moderately close together, in 

 a specimen 1.086 mm. long, 0.296 mm. apart. They are brownish black in 

 color. 



The maxillary shield is widest anteriorly, quite evenly rounded pos- 

 teriorly and with a prominent ancoral process, which is considerably 

 broadened at the tip, each lateral corner of which is produced to form a 

 recurved hook. The rostrum is moderate^ prominent. Mandibles 

 relatively short and broad, in a specimen 1.32 mm. long, their total 

 length being 0.24 mm. The basal segment is broadest posteriorly, where 

 its width is over two-fifths the total length; its ventral side is nearly 

 straight, with the posterior angle pi'oduced; its dorsal and posterior side 

 form together a sweeping curve from near the anterior end on the dorsal 

 side to the ventral posterior angle. The distal joint is rather heavy, but 

 the claw is deeply inserted and the exposed portion is relatively short 

 and slender, and quite straight. The area of oblique striae is rhombic in 

 outline and confined to the base of the claw, which is bent at a point even 

 with the distal margin of the segment, and ends in a sharp point. 



Palpi. — Comparatively small and only moderately thick. The first 

 segment is narrow, and 2 is wide, being twice as wide as 1, and wider than 

 long, measured from the middle of one end to the middle of the other. 

 Ventral margin slightly convex, the dorsal strongly so 3, short and mod- 

 erately broad with both sides nearly straight, while 4 is of moderate length. 



