1904.] ASELLOTA-GROUP OP CKUSTACEAXS. 306 



pair of pleopoda. Second pair is wanting, as in all Asellota. In 

 the male the first pair (PI. XX. fig. 2 g ; PI. XXI. fig. 2/) is 

 slightly longer and at the base narrower than the female oper- 

 culum ; the sympods of the two appendages are completely fused 

 with each other, forming a short, transverse plate ; each pleopod 

 has one ramus, which is free, oblong, and between two and three 

 times longer than the sympod ; each ramus can be moved a little by 

 a tiny muscle (^m.) in the sympod. The second pair is a good deal 

 smaller than the first ; each appendage (PI. XX. fig. 2 h ; PI. XXI. 

 fig. 2 g) consists of an oblong subtriangular plate, the sympod, the 

 inner margin of which is sinuate, and from the distal end of this 

 margin arise the two rami. The exopod is veiy small, slender, a 

 little curved, scarcely hook-shaped, one-jointed, but in S. siamense 

 (PL XXI. fig. 2 h, ex.) a vestige of a division into two joints is 

 observed. The endopod is rather long, very slender, two-jointed, 

 and strongly geniculate in the articulation (PL XXI. fig. 2 h, en.) ; 

 the proximal joint contains a muscle for the movement of the 

 second, which has no internal cavity, while its end is obtuse and 

 often furnished with a brush of exceedingly short bristles ; 

 especially in S. antillense, the terminal portion bearing this 

 brush is distinctly marked ofi" from the joint. The plate-shaped 

 sympod contains muscles to the rami, two to the endopod, and 

 at least one to the exopod. The two pleopoda of this pair touch 

 each other at their base ; they are covered by the first pair. In 

 both sexes the pleopoda of the fourth pair (PL XX. fig. 2 k) are 

 similar as to size and structure ; each has a short, broad sympod 

 and a two-jointed exopod, which is slightly longer and somewhat 

 broader than the un jointed endopod, and adorned with plumose 

 set£e along the distal part of the outer margin ; both rami 

 are lamellar and both seem to be respiratory. The fifth pair 

 (PL XX. fig. 2 I) has no discernible sympod and only one ramus, 

 which is large, un jointed, but otherwise shaped and .adorned with 

 setpe like the exopod of the preceding pair, and accordingly it is 

 in all probability the exopod itself. 



Uropoda consist of an unjointed sympod and two unjointed 

 nearly stylif orm rami ; the exopod is as long as or longer than the 

 sj^mpod, nearly as long as or somewhat shorter than the endopod. 



III. Coinpai'ison between the Genera Stenetrium and Asellus. 



PL 39 in Sars's work is filled with figures of Asellus aquaticusJj. 

 I can therefore refer to his good drawings, and give only a new 

 figure of the second male pleopod. 



The essential differences between Asellus Geofi". and Stenetrium 

 Hasw. are found in the antennae, the maxillipeds of the adult 

 female, and some of the pleopoda. The peduncle of the antennae 

 shows the same number of joints in both genera, but in Asellus 

 the exopod from the third joint is wanting. In Asellus the basal 

 joint of each maxilliped possesses in the ovigerous female a rather 

 large plate, bearing a number of bristles at the end and directed 



Proc. Zool. Soc— 1904, Vol. II. No. XX. 20 



