10 U.S. NATIONAL MUSEUM BULLETIN 266 



Segment 1 with anterior margin slightly or not expanded forward and 

 not divided into lobes, serrations, or quadrate areas 9 



6. Pores opening through special tubercles on or near the outer mar- 



gin of the keels STYLODESMIDAE 



Pores not on special tubercles but opening through channels in outer 

 margin of keels or from their upper surface 7 



7. Anterior border of segment 1 and outer margins of keels transparent, 



with opaque intrusions from inner surface . . . . » CRYPTODESMIDAE 

 Margins of segment 1 and of keels opaque, similar to inner surface . 8 



8. Outer and posterior margins of keels with prominent seta-bearing 



teeth PERIDONTODESMIDAE 



Outer and posterior margins of keels usually somewhat lobed but not 

 prominently serrate CH YTODESMIDAE 



9. Seminal canal of male gonopod widening into a distinct vesicle or 



ampulla near its distal extremity POLYDESMIDAE 



Seminal canal not distally widened or modified as above 10 



10. Lateral keels with pores on ventral surface; outer margins developed 



into two or three large acute lobes; outer margin of segment 1 lobed 



somewhat like keels TRIDONTOMID AE 



Keels with pores on outer edge or dorsal surface; outer margins not 

 developed into large lobes; segment 1 with outer margins not spe- 

 cially lobed 11 



11. Body shape suggestive oi Polydesmus; number of segments 18-21 ; 



living animals very active TRICHOPOLYDESMIDAE 



Body shape not suggestive of Polydesmus; segments 20, 19 in males 

 of Hypsiloporus (Comodesmidae); animals rather sluggish 11 



12. Body small, 10 mm. or less long; sides of segment 2 extending be- 



low those of segments 1 and 3; dorsum setose; coxae of gonopods 



large and enclosing much of distal joints COMODESMIDAE 



Size medium to large; segment 2 not extending below segments 1 and 

 3; dorsum rarely with conspicuous setae; coxae of gonopods not 

 enlarged or enclosing distal joints 13 



13. Pores on dorsal surface of keels which have rather thin, unrimmed 



outer margins that are irregularly dentate or tubercular. 



PLATYRHACIDAE 



Pores opening from more or less thickened outer rim of the keels 



which may have several small teeth but is not otherwise irregular 



or tubercular 14 



14. Last segment broadly rounded behind EURYURIDAE 



Last segment conical at apex 15 



15. Stigmata displaced anteriorly above the legs so that the posterior 



stigma is located above the anterior coxa, and the anterior stigma 

 is placed in the transverse segmental constriction in contact with 



the prozonite HOLISTOPHALLIDAE 



Stigmata not displaced, occupying the normal position 16 



