426 A HISTORY OF RECENT CRUSTACEA 



peraeon ; the face is sloping. The sides of the head are 

 distinctly marked by a vertical marginal line and an infra- 

 marginal line. The clypeus is arched.' The pleon has six 

 segments, of which the first two are narrower and usually 

 shorter than the third. The young quit the mother with 

 the seventh segment of the perason still undeveloped. 

 The first antennae are three-jointed. The second antennae 

 have the flagellum from two- to four-jointed. The first 

 maxillae have two plumose setae on the inner plate ; the 

 second maxillae have two plates ; the ' palp ' of the maxilli- 

 peds is two-jointed, the epipod oblong, acute. The trunk- 

 feet are rather long. In the first and second pleopods of 

 the male the inner branches form long narrow sexual 

 organs, those of the first pair often coalesced; in the 

 female the same branches are rudimentary, short, acute. 

 In the remaining pairs the inner branch is branchial ; in 

 all the pairs the outer branch is opercular, and often also 

 tracheal. The uropods are always prolonged beyond the 

 two terminal segments of the pleon. 



Budde-Lund, in his exceedingly valuable work on the 

 Terrestrial Isopoda, makes a family Onisci, which he divides 

 into two sections, Armadilloidea and Oniscoidea, but it 

 seems better to constitute two families, since it is the 

 almost invariable fate of large sections eventually to be 

 made independent. For the Oniscidae, or second section, 

 Budde-Lund gives a tantalising ' Conspectus Generum,' 

 based on the flagellum of the second antennae, on the 

 tracheal or non-tracheal character of the pleopods, and on 

 the uropods. 



I. Flagellum 2-jointed. 



A. With tracheae. 1. Porcdlio. 



B. Without tracheae. 2, Platyarthrus. 



II. Flagellum 3-jointed. 



A. With tracheae. 3. Scleropaetes. 



B. Without trache£e. 4. Oniscus. 



III. Flagellum 4-jointed. 



A. Uropods short. 5. Armadillonismis. 



B. Uropods elongate. 6. Deto. 



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