1872 ] WOODWARD EOCENE CRUSTACEA. 27 



tumid bases to relieve and diversify the surface of the carapace of 

 this elegant crustacean. 



Underside. — -We have six specimens of R. bispinosa, which exhibit 

 the underside in a more or less satisfactory manner. Of these, four 

 are males, and two are females, the difference in sex being indicated, 

 as in other Brachyures, by the disparity in the breadth of the ab- 

 dominal somites (PI. I. figs. 2, 3, 5, & 6). 



The Branchiostegal Plate. — The suture formed by the union of the 

 carapace with the branchiostegal plate is nearly parallel with the 

 latero-anterior margin, it then turns almost at a right angle at the 

 base of the great epibranchial spine ; and the plate rapidly becomes 

 narrower, ending at a point opposite that at which the limbs of 

 the fourth pair take their rise. The inner margin is deeply exca- 

 vated, to admit of the insertion of the legs along the margin of 

 the plastro-sternal plates. 



The Plastrosternum. — The five pairs of plates*, which are 

 visible and which, soldered together, compose the plastrosternum, 

 differ but slightly (save in their relative breadth) in the male and 

 female, the deep median sulcus being narrower in the former ( <$ ), 

 and broader in the latter ( 5 ), to admit the broader abdominal 

 plates of the female. It closely agrees in form with that of other 

 Portunidse. 



The Maxillipeds. — Three specimens of this crab show remains of 

 the external jaw-feet, or maxillipeds, more or less perfect. The 

 endopodite is broad, straight-sided, and divided by a suture near its 

 anterior third ; the surface is marked by a longitudinal'furrow ; the 

 exopodite is straight and narrow; both rise side by side, from a 

 common triangular basal joint. 



The three slender distal articulations of the maxilliped are not 

 preserved. 



The Abdomen. — The abdomen is composed of seven articulations ; 

 but the fifth and sixth joints appear to be soldered together in the 

 male. In the female the seven articulations are distinct, and in- 

 crease slightly in breadth from the first to the fifth, when they 

 again decrease, terminating in a broadly oval extremity. 



In the male, the seven articulations gradually but slowly decrease 

 in breadth to the seventh, which is bluntly rounded at its extremity. 



In the female, the first three segments are nearly linear ; but they 

 gradually increase in length as well as in breadth, to the sixth joint. 

 In the male the first two segments are also nearly linear ; but the 

 third and fourth are nearly equal in length to half their breadth, 

 whilst the conjoined fifth and sixth segments are proportionally 

 longer than wide. The seventh, or terminal segment, in the male, is 

 very small, as compared with the same segment in the female. 



The abdomen is without ornamentation ; but the caudal segments 

 are slightly trilobate in the female. 



* The Plastrosternum (which is homologous with the thoracic segments of 

 less highly cephalized forms) is really composed of seven plates ; but the two 

 most anterior, which bear the two pairs of maxillipeds or jaw-feet are very small 

 and are concealed beneath the matrix. 



