4g CRUSTACEA. 



still more distant. But there are Cancroidea that approach the 

 Grapsoid species in the distant eyes, and there are Grapsoidea Avhich 

 even exceed the Cancroid species in the approximation of the eyes at 

 base. This character, therefore, would place the two groups on nearly 

 the same level, or, we should rather say, that the grades are various 

 in both groups, yet the average character is somewhat higher in the 

 Cancroidea. The broader sternum and abdomen common in the Grap- 

 soids, is proof of the lower grade of the class. The outer antennae are 

 small, as in the Cancroids. 



In the LEUCOSOIDEA, we find the same narrow front, approximate 

 eyes, and small antennary space, as in the Maioids, and their charac- 

 teristics afford evidence of the high grade of the species. The more 

 perfect character of the efferent channel of the branchial cavity 

 appears to be a step beyond what is found in the other Brachyura. 

 The praelabial plate in the Macroura and ordinary Brachyura is essen- 

 tially the same; the improvement exhibited in passing from the 

 lower to the higher grade consists in its more perfect limits anteriorly, 

 and the more accurate adaptation of the outer maxillipeds to its borders. 

 Another step occasionally observed, is the division of it by a low ridge 

 separating an outer portion as the efferent channel. But in the Leu- 

 -cosoids, there is a higher perfecting of the branchial system, this chan- 

 nel being made a complete tube, through the modification of the proe- 

 labial plate its elongation in front at middle in conjunction with 

 the elongation and adaptation of a branch of the first pair of maxil- 

 lipeds. It is difficult to decide whether this peculiarity should be 

 admitted as proving a higher grade in the species, or only in the 

 branchial system alone. The existence of the ridge on the pr&labial 

 plate is not throughout a mark of superiority, since the Maioidea have 

 no such ridge, although unquestionably higher than the Eriphinse, in 

 which species such a ridge exists. All the facts, however, combine to 

 give the Leucosoids, especially the family Leucosidae, a high rank 

 but little inferior, we believe, to that of the Maioids. They have a 

 narrow, small abdomen, a well-compacted body, and often the hardest 

 shells that occur among Crustacea. The broad form of the Calappa is 

 only a lateral extension either side of the posterior part of the carapax. 



The CORYSTOIDEA, the only remaining grand division of the Brachy- 

 ura, has several marks of inferiority of grade. This inferiority is 

 strikingly seen in the large outer antennae, by which they approach 



