MARINE ISOPODA OF NEW ENGLAND, ETC. 305 



I.— ONISCEDiE. 



Antennulae rudimentary ; legs ambulatory ; pleon of six distinct seg- 

 ments, of which the last is small ; mandibles without palpi ; uropods 

 terminal.* 



This large and important group of Isopoda being terrestrial in habit, 

 only a few species are mentioned in this paper. They inhabit moist situa- 

 tions, and are commonly known as "sow-bugs," "pill-bugs," "wood-lice," 

 &c. Several species may often be found under an old board or pile of 

 rubbish. The genus Idgia Fabr. inhabits sea-shores, above tide-level, 

 and a few other genera are found under heaps of seaweed, or burrowing 

 in the sand along the shore. Three such species, belonging to as many 

 genera, are here described and figured, but are less fully treated of than 

 the marine species that follow in the other families. Other species, 

 especially of the genus Porcellio, may be found in similar situations. 



The family may be at once recognized by the apparent possession 

 of only a single pair of antennae. These are the antennae properly so 

 called, the autennuloe being minute and rudimentary. This is generally 

 regarded as a character indicating a high degree of development, and 

 causes them to somewhat resemble externally some of the shorter myrio- 

 poda, which, like other insects, have but a single pair of antennary 

 organs. The maxillipeds are large and operculiform in this family, with 

 short and few-jointed palpi. The mandibles are destitute of palpi. 



The legs are rather weak and fitted only for walking, and usually more 

 or less concealed by the projecting epimeral regions of the thoracic seg- 

 ments. The pleon, in our species, has its segments distinct and decreas- 

 ing rapidly in size to the last, which bears the more or less exserted uro- 

 pods. These organs may not, however, project beyond the general 

 outline of the pleon, as they scarcely do in Actoniscus, while in Armadillo 

 they assist in forming the very regular outline of that part of the body, 

 which closes against the head when those animals, as is their habit, roll 

 themselves into a ball on being alarmed. 



This family is placed by Bate and Westwood in a separate "division," 



the "^ro-spirantia," on account of their aerial respiration. The air, 



however, requires to be saturated with moisture, and in some of the 



genera the respiration is, in part at least, aquatic. On this subject the 



reader is referred to the publications of Duvernoy and Lereboullet and 



of Nicholas Wagner. 



Philoscia Latreille. 



PMloscia Latreillo, Hist. nat. des Crust, et ties Ins., tomo vii, -p. 43, "1804." 



Head rounded in front, not lobed ; antennae with its segments cylin- 

 drical, flagellum three-jointed ; pleon suddenly narrower than the thorax; 

 uropods exserted, basal segment broad, rami elongate. 



* The above diagnosis "would not include the genera Tylus Latreillo nor Helleria 

 Ebner, wliicli perhaps ought not to be regarded as belonging to this family, although 

 closely allied to it. 

 20 F 



