333 DISCOVERY REPORTS 



S. cornuta, his conclusions are justified ; but I may point out that even the smallest speci- 

 mens of S. cornuta exhibit the characteristic shape of the adult. It is interesting to note 

 that Studer does not state the sex of the specimen he figures ; nevertheless, Beddard says 

 it is a male, whilst Hodgson, referring to the same figure, says, "this figure, from its 

 great breadth is probably a female ". By this statement Hodgson recognizes one of the 

 chief difl^erences between the two species, but, as is usual amongst the SeroUds, the 

 male has the greater proportional width. The only other record of either species is that 

 given by Monod (1926) for S. trilobitoides, Eights. 



The most noticeable point of difi'erence between the two species lies in the relative 

 proportions of the various parts of the body. In both species, the male is broader in pro- 

 portion to its length than is the female . The following comparison is drawn between two 

 adult males. 



(i) The shape of the body of S. cornuta, excluding the terminal spine of the last 

 segment, is almost circular, whilst that of S. trilobitoides is broadly ovate ; the actual 

 measurements are: 



S. cornuta : length minus terminal spine = 49 mm. ; 



breadth = 49 mm. ; 



terminal spine = 4 mm. 

 S. trilobitoides: length minus terminal spine =52 mm. ; 



breadth = 48 mm. ; 



terminal spine = 2 min. 

 The proportions of the body in the specimens in the British Museum and also of 

 Hodgson's specimen agree with those of S. trilobitoides, whilst the measurements given 

 by Eights (70 x 57 mm.) for that species seem to point to the same general shape, 

 though the ratio of length to breadth is slightly greater, 1-22: i as compared with 

 I-I2 : I. Studer (1879) gives the length of his largest specimen of S. cornuta as 30 mm. 

 and makes no mention of its breadth ; but if the specimen is figured correctly his specimen 

 agrees in shape with those of the present collection, for the length without the terminal 

 spine is 32 mm., the breadth also 32 mm., and the length of the spine 3-5 mm. 



(2) The terminal segments of both species (Fig. 13 a, 6) are pentagonal in outline; 

 but that of S. cornuta is longer than broad with a much more acute posterior extremity 

 and with a longer terminal spine, whilst that of S. trilobitoides is broader than long ; the 

 measurements taken from two adult male specimens are : 

 S. cornuta: length of segment + spine = 19 mm. ; 



length of spine = 4 mm. ; 



breadth =17 mm. 

 S. trilobitoides: length of segment + spine = 18 mm. ; 



length of spine = 2 mm. ; 



breadth =19 mm. 

 This difference in shape can be clearly seen if the figures of Beddard (1884^, pi. i, 

 figs. 1-3) and of Hodgson (1910, pi. iv, figs, i, 2) are compared with those of Studer 

 (1879, taf. iii, figs. I, 3) for S. cornuta. 



