MARINE ISOPODA OF NEW PiNGLAND, ETC. 305 



I.— 0NISCLD2E. 



Antennulte rudimentary; legs amVjulatorj' ; pleon of six distinct seg- 

 ments, of wliicli tlie last is small; mandibles without palpi; uropodB 

 terminal.* 



This large and important group of I.sopoda being teiTestrial in habit, 

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

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

 &c. Several species may often b(j found under an old board or inla of 

 rubbish. The genus Ligia Fabr. inliabits sea-shores, above tide-level, 

 and a few other genera are found under heaj)s of seaweed, or burrowing 

 in the sand along the shore. Three such sprir;ies, belonging to as many 

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

 the mai-ine 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 i)air of antennse. These are the antennae properly so 

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

 regarded a.s a character indicating a high degree of development, and 

 causes them to somewhat rasemble 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 u.sually more 

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

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

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

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

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

 they assi.st 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-spLrantia," 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 refeiTcd to the publications of Duvemoy and Lereboullet and 

 of Nicholas Wagner. 



Philoscia •T^ati eille. 



PhUo9Cia Latreillc, HLst. nat. >\<-/ii Crast. et de3 Ins., tome vii, p. 4.3, ''1804." 



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

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

 uropods exserted, basal segmecit broad, rami elongate. 



* The above diagnosis would not include the genera Tylm Latreille nor Selleria 

 Ebner, which i>erhap3 ought not to be regarded as belonging to this family, althougli 

 closely allied to it, 

 " 20 F 



