HOMOLOGIES OF CRUSTACEA. 33 



the suture varies little in position in the Brachyura, still, as the 

 species become more elongated, there is often a deep depression, cor- 

 responding precisely in its course to that of the Macroural suture. 

 In the broad Cancridse, having an areolate carapax, the more strongly- 

 marked transverse depression is that which crosses just posterior to 

 the medial and the antero-lateral regions, and terminates just anterior 

 to the fifth normal tooth. But as the carapax elongates, this depres- 

 sion (which in all cases goes behind the medial region), instead of pass- 

 ing posterior to the antero-lateral regions, bends more forward, and 

 terminates anterior either to the second or third normal teeth. Even 

 in Eriphia, this depression has this forward course ; and in the Oxy- 

 rhyncha, which are more narrow oblong, the depression is often 

 strongly pronounced, and like the Macroural -suture in position, both 

 above and below, although never becoming a proper suture. The 

 carapax seems, in such cases, to be divided across very nearly as in 

 Astacus. 



These relations just pointed out, may seem to show that the suture 

 in the Macroura is actually homologous with the depression in the 

 Oxyrhyncha, rather than with the lateral suture of the carapax of 

 these species. It certainly proves a similarity of position in the two ; 

 yet we are still disposed to infer, that the lateral suture of the Bra- 

 chyura is actually represented by the transverse suture of the Ma- 

 croura. The latter have no lateral suture, and the transverse suture 

 commences at the same point in each tribe. The Oxyrhyncha indi- 

 cate, by the character mentioned, a tendency which is exhibited in a 

 developed condition in the Macroura. It is a foreshadowing of a 

 structure which is not consistent with the Brachyural type, but which, 

 when the abdomen is prolonged, as also the cephalothorax (as in 

 Pagurus or Astacus), becomes characteristic of the body. 



More direct evidence, with regard to the normal constitution of the 

 Macroural carapax, is obtained by tracing the transitions through the 

 Thalassinidea and Astacus to the other Macroura. The longitudinal 

 suture, called the epimeral by Edwards, actually exists in most Tha- 

 lassinidea; and besides, there is a transverse suture crossing the 

 back, as in Astacus, connecting nearly the middle points of the longi- 

 tudinal sutures.* The longitudinal sutures are nearly horizontal 

 in Thalassina, but more oblique in Gebia, very much as in Astacus. 



* See, in the Atlas, the Plate illustrating the Thalassinidea. 



9 



