351 



19. Abdomen without transverse suture. . . . Trhypochthonius. 

 Abdomen with transverse suture 20. 



20. Ungues tridactyle, with inner dactyle less than half 



as long- as the others; cephalothorax truncate in. 



front Parhypochthonius. 



Ungues either monodactyle or tridactyle. if tridactyle, 

 dactyles of equal length; cephalothorax not truncate in 

 front Hypochthonius. 



KEY TO THE GENERA OF HOPLODERMIDvE. 



1. Genital and anal openings situated apart; ventral plate 



large Mesoplophora. 



Genital and anal openings situated together; ventral plate 

 small or rudimentary 2. 



2. Ungues monodactyle; genital and anal covers sepa- 



rate HOPLODERMA. 



Ungues tridactyle; genital and anal covers coalescing. 

 Phthiracarus. 



Descriptions of Species. 



Thirty-three species from Illinois are described in the 

 following pages, and of this number twelve are new. In every 

 case the author has made figures illustrating the new species, 

 and a few figures have been made of some of our most common 

 American species. The species are arranged in the natu- 

 ral order under each genus, a key being given to aid in 

 their identification. The genera and families are similarly 

 arranged, the leading characters being given for each group. 



Family ORIBATID^. 



Forms with the cephalothorax and abdomen immovably 

 fused together; body not compressed; mouth-parts very small; 

 tracheae opening at the acetabula of the legs. Abdomen 

 possessing chitinous wing-like expansions called pteromorphse; 

 integument usually smooth and shiny. Legs never with 

 swollen or moniliforni segments. 



