462 ONISCID^. 



ISOPODA. ONISCIDJE. 



NORM A LI A. 



Genus— PLATYARTHRUS. [Brandt.) 



Generic character. Flattened, broadly ovate, scabrous. 

 Cephalon transverse. Lateral and frontal lobes strongly 

 developed, arising from the anterior margin. Outer antennse 

 apparently six-jointed; second joint small; third short, nar- 

 rowed outwardly at its base ; fourth, ovate ; fifth, largest, 

 broad, flattened outwardly, but narrowed at its base ; sixth, 

 thin, pointed at the tip, with a slight indication of a joint at 

 its base. Terminal feet with the basal portion large, exposed. 

 Terminal portion as long as the basal, conical. Inner appen- 

 dage slender, exserted. 



The type of this genus is closely allied to Oniscus 

 and Porcellio, but differs in its short flattened antennae, 

 scabrous, perfectly opaque, white body, as well as in 

 various minor structural details. The head has its 

 anterior lobes arranged nearly as in Armadillo, being 

 derived, according to Dr. Kinahan, from the ce.phalic, 

 mandibular, or fourth ring of the head, and not from 

 the third or second, i.e., the antennary rings. The third 

 is but badly developed and small, but the distinction 

 between it and the fourth ring is evident. 



The internal antennas are three-jointed and very 

 minute ; the outer antennas are short but broad, flat, 

 and scabrous, the fifth joint being much the largest, 

 and dilated. We have not been able, with a powerful 

 lens, to discover any articulation at the base of the ter- 

 minal joint, as described by Dr. Kinahan, although a 

 slight dilatation in the lateral maryin near the base seems 



