604 BULLETIN 159, UNITED STATES NATIONAL MUSEUM 



KEY TO THE GENERA OF THE SUBORDER MONSTRILLOIDA 



(Owing to the absence of second antennae and mouth parts and the similarity 

 in structure of the first antennae and swimming legs, distinctions in this group 

 are based on the form and segmentation of the body and the armature of the 

 caudal rami) 



1. Body cyclopoid, metasome much wider than urosome; fourth and 



fifth legs rudimentary and of similar structure; first antennae 



9-segmented, setose; 60 fathoms 9 Thespesiopsyllus, new name ^s 



Body elongate, cylindrical, urosome nearly as wide as metasome; 

 fourth legs as well developed as first 3 pairs; first antennae 

 4-segmented, or less 2 



2. Urosome of female 3-segmented, of male 4-segmented 3 



Urosome of female 2-segmented, of male 3-segmented 4 



3. Oral tubule near the center of the ventral surface of the cepha- 



lothorax; fifth legs present in male; caudal rami with 5 or 6 



setae; surface tow c3^ ? Monstrilla Dana, 1848 (p. 393) 



Oral tubule placed far forward near anterior margin; fifth legs 

 absent in male; caudal rami with 2 apical and 2 lateral setae; 

 surface tow cr' ? Monstrillopsis G. 0. Sars, 1921 



4. Caudal rami club-shaped, each with 3 setae in female, 4 in male; 



oral tubule at anterior margin; fourth legs in female shortened; 



surface tow c?" 9 Cyrabasoma Thompson, 1888 (p. 395) 



Caudal rami linear, each with 5 setae in female; oral tubule 

 removed from anterior margin; fourth legs as long as other 

 pairs; surface tow 9 Thaumaleus Kr0yer. 1849 



KEY TO THE GENERA OF THE SUBORDER CALIGOIDA 



(Important characters are still furnished by the structure of the 5 pairs of legs 

 and the second antennae, but they are largely supplemented by the relative 

 size, shape, and armature of the various body regions) 



1. Entire body more or less distinctly segmented and depressed; 



fourth segment narrowed; genital segment enlarged, often 



to the size of the carapace 2 



Body more or less distinctly segmented and cylindrical; fourth 

 segment as wide as third; genital segment usually smaller, 

 rarely enlarged 47 



Body cylindrical but not segmented; head and neck distinct, 

 with horns or processes for attachment; rest of body fused 

 into a trunk 68 



2. Three anterior segments fused with the head; first or fourth legs, 



or both pairs, often uniramose 3 



Two anterior thoracic segments fused with head; first 4 pairs of 

 legs biramose; no dorsal plates; genital segment never as large 

 as carapace; outside of fishes cf 9 Trebius Kr0yer, 1838 (p. 413) 



First segment only fused with head; first 4 pairs of legs biramose 



(except Pholidopus) ; females with, males without, dorsal plates 23 



«8 Sars proposed the name Thaumatopsyllus for this genus, but that name was published by T. Scott In 

 1894 as a synonym of Aegisthus. 



