MARINE CRUSTACEANS. 



425 



are opposed to a group containing the Oxystomes together with the rest of the crabs. But 

 quite apart from this consideration is the fact that the Oxystomata have certain well-defined 

 features which give them a unity such that it would in any case be necessary to oppose 

 them to the rest of the Genuina; and, if this be not done by raising them to the rank 

 of the Dromiacea, then a new grade of classification must be introduced within the Genuina, 

 which must be separated into Oxystomes and non-Oxystomes. Such a step would be most 

 undesirable in view of the already complicated system of the Decapoda. 



At the same time I am unable to follow Ortmann in raising the three groups of crabs 

 to the same rank as the Astacidea or Paguridea. The crabs are a true group with a 

 common phylogenetic origin from Macrurous forms of the Reptantia through the Dromiacea. 

 Moreover, besides the shape of the body, there are three features which are almost absolutely 

 diagnostic of the Brachyura in the old sense of the word: (1) the loss of the last pair 

 of abdominal limbs, (2) the fusion of the 

 shield of the cephalothorax with the epistome 

 in such a way as to form two sockets for the 

 sense organs of the two sides, and (3) the 

 broad, unjointed endopodite of the first maxil- 

 liped, with its marked outer angle (Fig. 110). 

 In these circumstances I follow Boas in keep- 

 ing all the crabs together as a single tribe, 

 Brachyura, of the Reptantia. If we thus use 

 the word in its old, full sense, it becomes 

 needful to find another name for Ortmann's 

 Brachyura (= Boas' Brachyura genuina without 

 the Oxystomata). For this group I have al- 

 ready proposed • to use de Haan's name 

 Brachygriatha. 



I now pass to the consideration of the 

 subdivisions of the Brachygnatha. The Oxy- 

 rhyncha fomi a natural and, on the whole, 

 well-defined group, but a boundary between 

 the Cyclometopa and the Catometopa is ab- 

 solutely wanting. This is especially the case 

 between the Gonoplacidae and the Xanthidae, Fig- HO. The first maxillipeds. A. Calappa, B. Portuims, 



where the transition is made complete by C. Dromia,D. Cardiosoma,E. Cancer, F. Uca ( = Gela- 



, _., , J ni J -I Si'mHs), «. the inner lobe characteristic of the Portunidae, 



such genera as Pilmimoplaa: and Platypilum- ^ j^^ ^^^^^ ^^g,^^ weU-marked in crabs. 



nus. The Potamonidae also are far from pre- 

 senting an easy problem in their affinities. And the genus Catoptrus, while it seems clearly 

 allied with the Carcinoplacinae, is extraordinarily like the Portunidae, this likeness extending 

 even to the form of the first maxilliped, which is peculiar to the latter family (B. Fig. 110). 

 In view of these facts I propose to unite the Cyclometopa and Catometopa into a single 

 group, for which I propose the alternative titles Brachyrhyncha and Cancroidea", corresponding 



' Proc. Zool. Soc. 1900, p. 568. This term iu its original - This word was used by de Haan (in von Siebold'a 



meaning included the Dromiacea as well as the Cancroid "Fauna Japonica") with practically the same meaning as 



forms, but it has so long been in disuse that its revival in a that given to it here, 

 somewhat altered sense would cause no inconvenience. 



