DESCRIPTION OF SPECIES 



323 



(3) The spines on the median dorsal keel of the terminal segment are much larger 

 and fewer in number in S. cornuta than in S. trilobitoides. 



(4) The length of the coxal plates of the seventh thoracic somite also differs in the two 

 species. In S. cornuta (Fig. 13 Z)) they extend back- 

 wards almost to the level reached by the tip of the 

 terminal segment, and a considerable distance 

 beyond that reached by the pleural plates of the 

 second and third abdominal segments ; the pleural 

 plates of the second segment are slightly longer 

 than those of the third, and both pairs extend back- 

 wards beyond the distal end of the protopodite 

 of the uropod. In S. trilobitoides (Fig. it, 0) the 

 coxal plates reach backwards to a level half-way 

 between the attachment of the uropod and the tip 

 of the terminal segment and a little way beyond 

 the pleural plates of the abdominal segments : the 

 pleural plates of the second abdominal segment 

 are very slightly longer than those of the third. 



The British Museum specimens of " Serolis 

 cornuta" agree with S. trilobitoides on this point, 

 as do also the figures of Hodgson (1910, pi. iv, 

 figs. I, 2), though those of Beddard (1884 b) show 

 the coxal plates of the seventh thoracic somite in 

 the male (pi. i, figs, i, 3) rather more than their 

 normal length though not as long as those of the 

 male in the true S. cornuta. Studer (1879), i^^ his 

 original description of S. cornuta, states that the 

 coxal plates of the seventh thoracic somite extend 

 beyond the middle of the caudal shield, and in his 

 figure (taf. iii, fig. i) shows them extending to the 

 base of the terminal spine, the level reached by the 

 corresponding plates in the females of this collec- 

 tion. In his figure of the abdomen (taf. iii, fig. 3), 

 which also includes these plates, they are shown 

 very much shorter, whilst the pleural plates of the 

 abdominal segments do not reach as far as the level 

 of attachment of the uropods. Since in all existing 

 figures, and in all examined specimens of both species, the pleural plates extend back- 

 wards beyond the level of the articulation of the uropod, it may be presumed that on 

 these points Studer's figure (fig. 3) is inaccurate, though the actual form of the terminal 

 segment appears to be correct. 



(5) Observations made on living specimens of the two species show that a further 



Fig. 



It. 



13. Seroh's trilobitoides. Eights, and 

 S. cornuta, Studer. 

 S. trilobitoides. Eights, a, abdominal and 

 terminal segments : x 2. 

 S. cornuta, Studer. b, abdominal and ter- 

 minal segments: x 2. c, cutting edges of 

 mandible: x 40. 



