532 CRUSTACEA. 



non palpigerse, supra profunde bipartite, processu apicali oblongo, 

 angusto. 



2. Maxillipedes 2di tenuiterpediform.es. 



Fam. IV. PASIPH^EID^E. Mandibulje uti in Atyidis. 



It will be observed, that the Alpheus group is not a separate divi- 

 sion in this classification, but is included with the Paltemonidoe. 

 Among the characters Usually given to it, there are none that are 

 important as distinctions, and any application of them is full of ex- 

 ceptions and difficulties. The absence of a beak in the genus Alpheus 

 gives a peculiar appearance to the species ; but this character does 

 not belong to the other accepted Alpheidae. The genus Pontonia is 

 closely related to Palaemon, much more nearly than to Alpheus. A 

 depressed form and short beak characterize some of the species; but 

 from these there is a transition by slight shades, of difference, to 

 Palsemon. In both groups, the hands of the second pair of legs are 

 stout, whereas, in Alpheus, they are filiform and annulated. 



In the farther subdivision of the families into subfamilies, we deem 

 it of the first importance to regard the relative character of the first 

 and second pairs of legs. The distinction between them is so wide 

 and unvarying, through all the Brachyura, that characters based on 

 the differences, cannot be of small value among the Macroura. We, 

 therefore, place the species having the first pair the stouter in a sepa- 

 rate subfamily from those in which the second is the stouter. 



We here present a synopsis of the families, subfamilies, and known 

 genera. 



FAM. I. CRANGONID^!. 



S0BFAM. 1. CRANGONIN^E. Pedes Imi 2dis crassiores. Maxilli- 

 pedes externi pediformes. Digitus mobilis in manus marginem 

 claudens, immobilis spiniformis. Pedes 2di non annulati. 



Gr. 1. CRANGON, Fair. Rostrum brevissinmm. Oculi liberi. Pedes 2di chelis 

 armati, 4ti 5tique acuminati, gressorii. 



