Imhim I }i('p-sf'i( DrcJiii'iKj, 2^7 



a distinct marginal raisi'd rim cxti'iids from the .subbranchio- 

 stogal spines backwards on each side, increasing; towards tliR 

 posterior margin, being especially well-marked posfcerolate- 

 rally, where it rises into a strong and bold ridge, forming at 

 each end of the carapace the posterior boundary of a deep 

 groove; tlie ridge with the groove concentric therewith con- 

 stituting the thoracic element of a sti'ong thoracico-abdoniinal 

 liinge. 



The branch iostegites are abruptly inflected, and their fr(;c 

 margins, which are closely ajiplied to the bases of the legs, 

 are widely but obtusely angulated inwards opposite to the 

 interval between the first and second ))airs of legs anteriorly, 

 while ])Osteriorly they give off a triangular process which 

 abuts against the posterolateral face of the eighth thoracic 

 sternum, and thus serves not only to keep the two elements 

 of the thoracico-abdoniinal hinge in constant relation of appo- 

 sition with one another, but also to divide that which answers 

 to the atJerent branchial cleft in Antaeus into two parts, an 

 inferior and a superior: in the former of these the free margin 

 of the carapace is in such close contact with the leg-b.iscs as 

 to leave no passage for water to enter; the latter, on the 

 contrary, is a wide and rigidly-patent oval aperture j)hicing 

 the branchial chamber of its own side in direct communication 

 with the subabdominal cavity, and forms the exclusive inlet 

 for the water required for respiration : whence it follows that 

 all the water which enters the branchial chambers must do so 

 by way of the subabdominal cavity, and that during lite a 

 constant circulation must be maintained in this cavity; in 

 the female, in which the special afferent branchial apertures 

 are larger than in the male and tiie subabdominal cavity 

 forms a spacious brood-pouch, the constant circulation of 

 water in the latter must secure a more perfect aeration of the 

 eggs than would otherwise occur; there is no doubt, in fact, 

 that we have here to do with a mechanism for securing the 

 due aeration of the egii's similar to that which exists iii 

 Encephahides Armstroiuji and other deep-water Brachyura 

 (Ann. & Mag. Nat. Hist. (6), vii. pp. 259, 266, et 267j, 

 wherein the branchial cavities communicate with the brood- 

 cavity by means of canals in the hinder angles of the cephalo- 

 thorax and, the ordinary direct channels being closed, water 

 for respiration is derived from the brood-cavity. 



The rostrum is compressed, and presents four longitudinal 

 spiny ridges — one dorsal, two lateral, and one ventral ; the 

 spines of these are all sharj), slender, forwardly curved and 

 inclined, and decrease in length from the base towards the 

 obsoletely bitid apex of the rostrum. The dorsal ridge is 



20* 



