new BracJii/urous Crustacean. 539 



is pvodnced into a slioit blunt spine ; next tlie articulation of 

 the larger palm witli the dactylus there is a large lohe or 

 tooth directed towards the end of the dactylus, as in Sajlhi 

 and Gatunia. The digits on their prehensile edges have 

 rather large and closely-set irregular teeth, and the bas;il 

 tooth in the larger dactylus is much enlarged and directed 

 obliquely backwards as in Scylla and Gatunia. 



The tiiree pairs of ambulatory legs have the segments 

 somewhat flattened ; the merus is comparatively long, and 

 the carpus and succeeding segments are very distinctly 

 grooved along their thick anterior margins, the posterior 

 margins being thinner and narrowly rounded. The legs of 

 the last pair are flattened and broadened to serve as swimming 

 organs ; the dactylus, which is slightly incompletely pre- 

 served, is broadly ovate and leaf-like, and the propodus is 

 expanded and thinned only on the posterior side, in conse- 

 quence of which the articulation with the dactylus is close to 

 the stouter anterior edge. 



Affinities. — One of the most conspicuous characters of this 

 crab — a feature hitherto unknown earlier than Tertiary times 

 — is the modification of the last pair of legs into swimming- 

 paddles resembling very closely those found in the Cyclo- 

 metopan family Portunidre. This resemblance does not 

 consist merely in the fact that the segments of the limb are 

 broad and flattened : the broadly ovate leaf-like dactylus, the 

 propodus exjninded and thinned only on the ventral or poste- 

 rior side, so that the articulation with the dactylus is close to 

 the stouter dorsal or anterior edge, and the general outline of 

 these and other segments, are closely paralleled in the 

 swimming-paddle of such a form as J^eptuyius. The per- 

 sistence of this type of limb throughout the large and varied 

 family Portunidie might well lead us to regard it as a cha- 

 racter of great phylogenetic significance. It reappears, 

 however, with almost identical form in the genus Matutu 

 among the Oxystomata, whose community of inheritance with 

 the Portunidaj is very improbable indeed. 



The paddle-like extension of the posterior legs is clearly an 

 adaptive character, and may well have been acquired inde- 

 pendently by different branches of the Brachyuran stock. 

 This therefore throws no light on the affinities of our present 

 crab. 



We consequently have to rely on the structure of the 

 carapace. This is square and flat, and its marked features 

 are the dcflexed and trilobed front, the wide stretch marked 

 off into wide rounded teeth on either side of the front, and 



