104 PROCEEDINGS OF THE NATIONAL MUSEUM vol.84 



7-JOINTED AND 8-JOINTED LEGS 



In Spirostrephon, Diactis, and Tynomma all the legs are 7- jointed, 

 being composed of a basal or coxal joint, a very short second joint, a 

 third joint longer than any of the three succeeding joints, which are 

 subequal, and a seventh or outer joint distinctly longest of all. In 

 Colactis and Heptium some of the anterior legs are 7-jointed, but all 

 the remaining legs have an eighth joint, which is formed by a division 

 of what corresponds to joint 7 of the first few legs. In these two 

 genera the transition to 8- jointed legs is not always distinct, since in 

 some females it obviously occurs on the third pair of legs, while in 

 other females the eighth joint is not observed with certainty until 

 somewhat farther back. In the males the first three pairs of legs 

 are plainly 7-jointed, and in the species with the next four pairs of 

 legs lacking ventral pads on the last joint the legs usually are seen 

 to be 8-jointed. In males of the species with ventral pads on the last 

 joint of some of the anterior legs, the transition to eight joints ap- 

 pears to take place gradually and somewhat farther back, so that 

 only the last few pairs of pad-bearing legs and the legs thereafter 

 may show unmistakably the eighth joint. 



SYSTEMATIC TREATMENT 



KEY TO THE NORTH AMERICAN GENERA OF LYSIOPETALIDAE 



1. Segment 1 with 20 crests; anterior segments with secondary 



crests scarcely smaller than primary crests Spirostrephon 



Segment 1 with less than 20 crests; secondary crests distinctly 



smaller than primary crests on all segments 2 



2. Transition to full number of dorsal crests occurring on segment 8 



or 9; segments 2, 3, and 4 with crests distinctly oblique and 

 divergent, inner primary crests close to median line in front, 

 widely separated at posterior margin; males with a small acute 

 process projecting forward from basal joint of anterior pair of 



legs on numerous segments near middle of body Diactis 



Transition to full number of dorsal crests occurring on segment 

 11 or later; segments 2, 3, and 4 with crests nearly parallel, or 

 median crests slightly diverging backward; males without 

 special processes on any of legs near middle of body 3 



3. Transition occurring on segment 11 or 12; segment 1 with 16 to 



18 crests; secondary crests as long as primary crests, reaching 

 posterior margin of segments and remaining distinct on 



anterior segments Tynomma 



Transition occurring on segment 16 or beyond; segment 1 with 

 10 crests; secondary crests notably shorter than primary crests, 

 not reaching posterior margin of segments, usually obsolete 

 on segments 2, 3, and 4 4 



4. Transition occurring on segment 16 or 17; males with seventh 



pair of legs of normal size and structure C olactis 



Transition occurring on segment 18 or 19; males with seventh 

 pair of legs greatly reduced in size and with a long erect spine 

 on coxa Heptium 



