REPORT ON" THE ISOPODA. 9 



joint of peduncle. Mandibles provided with a palp. Maxillipedes expanded, operculi- 

 form ; first pair of thoracic limbs (in the male only ?) with a large prehensile manus, the 

 penultimate joint swollen and subtriangular in form, with a row of serrate spines along 

 the anterior margin which correspond to a row of similar spines upon the last joint ; 

 remaining pairs of limbs slender, ambulatory, with a double terminal claw. Caudal 

 appendages biramous, inserted on the border of the shield-like abdomen near the 

 extremity." 



The genus Stenetrium is closely allied to Ianthe, Asellus, J&ra and Janira; it 

 agrees with all these genera in the equality of size of the thoracic segments, differing hi 

 this respect from such genera as Pleurogonium, Ischnosoma, &c. 



The genus has, however, been rightly distinguished by Haswell. The main points in 

 which it differs from any of the above mentioned genera are as follows : — from Asellus it 

 differs in the presence of two terminal claws on the ambulatory limbs, and in the fact 

 that the male has only one pair of thoracic appendages modified into a prehensile hand 

 instead of two ; also, the existence of two free abdominal segments in Asellus is a 

 character not found in Stenetrium. All the other genera mentioned agree with Stenetrium 

 in having two terminal claws, while Ianthe and Janira show a still farther point of 

 resemblance in the presence of a rudimentary exopodite on the antennae; x Stenetrium is 

 distinguished from Janira, by the powerful first thoracic limbs of the male, a character 

 which does not appear to be found to so marked an extent in any other Isopod. From 

 Iolanthe the latter characteristic sufficiently distinguishes Stenetrium, as well as the com- 

 paratively flattened depressed body and the form of the abdominal appendages 

 (compare PI. IV. fig. 13, with Bovallius, he. cit., pi. iii. figs. 29, 30, 32, 33, 34). 



Of this genus four species are now known. Two species, Stenetrium armatum and 

 Stenetrium inerme, have been described by Haswell from Sydney, Australia, in shallow 

 water ; a third species was dredged by Chilton in Lyttelton Harbour, New Zealand, and 

 named by him Stenetrium fractum; the fourth species, Stenetrium haswelli, will be 

 described in the following pages. It is the only one of the genus known from deep 

 water, and it bears out one generalisation that has been arrived at from a study of the 

 deep sea fauna, viz., that the inhabitants of deep water are larger than their shallow-water 

 allies. 



Stenetrium haswelli, F. E. Beddard (PL IV. figs. 1-8). 



Stenetrium haswelli, F. E. Beddard, Proc. Zool. Soc. Lond., 18S6, pt. i. p. 103. 



This species, as already mentioned, is represented by a single example, dredged in 600 

 fathoms, off the east coast of South America. 



1 This fact has been pointed out by J. E. V. Boas {Morphol. Jahrb., Bd. viii. p. 493 note, 18S3) in the case of 

 Janira; Bovallius (Biliang til Svensk. Akad. Eandl., Bd. vi., No. 4, pi. i. fig. 7) has depicted the articulated squarue of 

 Iantlie speciosa, while Mr. Chilton has noted and figured (loc. cit, pi. xviii. fig. 3&), the same structure in Stenetrium fractum. 



(ZOOL. CHALL. EXP. PART XLVIII. 1886.) Bbb 2 



