168 CEYLON PEAEL OYSTER REPORT. 



for the present, to frame a fresh definition, it may be pointed out that the species 

 described below, which are referred to Oyclaspis on the ground of general similarity 

 in form to species already included therein, differ from them in the fact that they 

 possess, in the adult female at least, five distinct thoracic somites behind the carapace. 

 Since, however, the first somite is entirely hidden in most cases in the adult male as 

 well as in immature specimens of both sexes, it seems inadvisable to make this 

 character the ground for separating these sjjecies generically from those in which the 

 somite in question is concealed or suppressed in both sexes at all stages of growth. 



Cyclaspis costata, n. sp. Plate III., figs. 39 to 53. 



Description of adult Female. Total length 375 millims : 



Carapace about three-eighths of total length, its vertical height nearly two-thirds 

 of its length, compressed, the dorsal surface rounded posteriorly, but keeled 

 in its anterior half, where there is a well-marked depression on either side of the 

 middle line. Pseudorostrum acute, prominent and slightly upturned as seen from 

 the side. Antenna! notch shallow and widely open, the antennal tooth obtuse. 

 Ocular lobe large, sub-circular, pseudorostral plates meeting in front of it for a 

 distance equal to its diameter. The eye pigmented and apparently well developed. 

 The sides of the carapace, behind and below the depressions mentioned above, are 

 beset with longitudinal ridges formed by rows of minute granules. There are about 

 twelve such ridges on either side in the specimen figured, with fainter secondary 

 ridges interposed between some of them. In a larger female specimen the ridges are 

 more numerous and closer together, owing apparently to the greater prominence of 

 the secondary ridges. 



First leg-bearing somite well exposed. Second somite produced dorsally into a 

 median crest. Fourth and fifth having the dorsal surface raised into a rounded 

 tubercle on each side of the middle line. 



Abdomen shorter than the cephalothoracic region, the somites comparatively stout, 

 with a faintly indicated dorsal keel, and with well-developed " peg and socket " 

 articulations laterally. 



Antennules (fig. 42) very short, first segment of peduncle much enlarged and 

 longer than the other two segments together, outer flagellum with two segments, 

 inner flagellum very minute. 



Antennae (fig. 43) unjointed, not produced, bearing two plumose seta?. 



The mouth-parts closely resemble those of C. australis ; the mandibles bear about 

 13 spines. The branchial apparatus was not examined. 



The third maxillipeds (fig. 44) have the basis abruptly bent at about the middle 

 of its length, and slightly longer than the succeeding segments together. The 

 external process of the merus is very large, .extending far beyond the process of the 

 basis. The propodus is expanded, not much narrower than the carpus, and the 

 dactylus is almost rudimentary. 



