144 CRUSTACEA. 



grand divisions ; one of which may be called the typical, another the 

 Qrapsoid, and the third the Corystoid division, as follows : 



Legio I. CANCRINEA, or Cancroidea typica. 



Legio II. TELPHUSINEA, or Cancroidea Grapsidica. 



Legio III. CYCLINEA, or Cancroidea Corystidica. 



The second and third divisions contain but few genera. The first 

 includes several families and subfamilies, based on important charac- 

 teristics. 



The structure of the efferent canal leading from the branchial 

 cavity, as already explained, has a high value in classification. By 

 means of it, the Leucosoidea are separated from all other Brachyura; 

 the distinction has been shown to be wide, and to attend striking cha- 

 racteristics of other kinds. The inner branch of the first pair of 

 maxillipeds undergoes a corresponding modification, and, therefore, 

 becomes itself important as a means of distinction. 



Among the Cancroidea, there are peculiarities of a somewhat analo- 

 gous kind. Although the efferent passage covers uniformly the outer 

 part of the pnelabial plate or palate, it is in some species found with 

 better defined limits than in others. In a large number of genera, 

 there is no separation from the rest of the pnelabial surface ; but in 

 other genera there is a longitudinal ridge, giving this canal distinct 

 bounds. This ridge is very perfectly developed in Eriphia, and the 

 narrow inner branch of the first maxillipeds covers the canal as in 

 the Leucosoidea. It is equally complete or nearly so, in Ruppellia, 

 CEthrus, Ozius, a genus separated from Xantho on this ground, and in 

 several other genera. It is distinct also in Pilumnus. But in Cancer, 

 Xantho, and many of the ordinary genera, there is no such ridge, or 

 if traces of it exist (as in Pseudocarcinus Rumphii, some Carpilii and 

 others), it is short, and does not extend to the front margin of the 

 palate. This character affords therefore a natural division among the 

 Cancroidea, though not the sole character at the basis of a natural 

 classification. 



The natatory character of the posterior legs is another important 

 characteristic. 



Moreover, among the natatory species, De Haan has pointed out a 

 character of much value : that in many of them the inner branch of the 

 outer maxillipeds has a small lobe attached to the inner margin, 

 which lobe is wanting in the rest of the swimming species, and in all 



