ORDER ISOPODA — ONISCUS. 51 



GENUS ONISCUS. Linneus. 



Body oval, with transverse crustaceous subimbricated segments, susceptible of being rolled 

 into a ball. The two outer antennae very conspicuous, setaceous, bent, of eight joints ; 

 the inner antennae obsolete. Two sessile eyes. Two prominent caudal appendices. Ter- 

 restrial. 



Oniscus asellus. 



PLATE VI. FIG. 18. 



Oniscus asellus. Lin., Fab., Lat., Desh. Consid. Gen. p. 320, pi. 49, fig. 5. 

 O. affinis. Say, Journ. Acad. Nat. Sciences, Vol. 1, p 430. 



O. asellus. Gould, Invertebrata of Massachusetts, p. 366. 



Description. Body oval, and roughened on its anterior portion. Segments of the body 

 rounded in front on the lateral edges, and pointed behind. Tail smooth, of six segments 

 (overlooked by the lithographer in the figure) ; the third, fourth and fifth with lateral prolonga- 

 tions ; the sixth or last segment pointed with four styles, longer than the last segment. Feet 

 fourteen, gradually increasing in size from before. 



Color, dusky brown, with many irregular ashen points and marks ; beneath greyish 

 white: 



Length, 0-5. Breadth, 0-3. 



This is the common Sow-bug of cellars and gardens, and found also under stones and de- 

 caying wood. The female carries its eggs in an oval sac beneath the body, where they are 

 hatched. The Sow-bug was formerly employed in medicine as a diuretic, but is now very 

 properly abandoned. It feeds on decomposed vegetables, and is in no wise injurious to 

 man. I coincide with Dr. Gould in considering the affinis of Say to be identical with that of 

 Europe. 



GENUS PORCELLIO. Latreille. 



Body resembling that of the preceding genus, with the same characters, except that the outer 

 antennas are composed of but seven articulations. 



Obs. This genus has been united by most recent writers with the preceding. 



PORCELLIO SPINICORNIS. 



Porcellio spinicornis. Say, Jour. Acad. Nat. Sciences, Vol. 1, p. 431. 



Description. Body elongate-oval, roughened with numerous granules. Third joint of the 

 antenna? elevated above, and armed with an acute spine. Terminal joint of the tail canali- 

 culate, hardly surpassing the first joint of the outer styles. 



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