188 Mr. J. Wood-Mason on a new 



2. Maxillipedes externi late hiantes. 



Gen. 3. Cardisoma, Latr. Articulus maxillipedis externi quartus 



apice tertii externo insitus. 

 Gen. 4. Gecarcoidea, M.-Edwards. Articulus maxillipedis externi 



quartus marginis medio excavato apicalis tertii insitus. 



Subfam. II. GECARCINIISLE. Articulus maxillipedis externi quartus 



et sequentes tertio celati. 

 Gen. 1. Gecarcinus, Latr. 



This division is unnatural, as separating the genus Gecar- 

 coidea (liodie Pelocarcinus) from Gecarcinus, to which it is 

 most closely related, and ranging it with others with which 

 its relations are more general ; and the classificatory value 

 of the character upon which it is based is moreover much 

 diminished, if not altogether destroyed, by the discovery of a 

 new form presenting an interesting transition from the former 

 to the latter genus in this very character. A more natural 

 result can, however, be attained, and Dana's subfamily names 

 still retained, by the substitution of another maxillipedary 

 character for the one originally selected, and now proposed to 

 be rejected. Gecarcinus, Pelocarcinus, and Hylaiocarcinus } in 

 fact, agree with one another, and differ from all other genera of 

 the family, in that the exopodites of their outer foot-jaws are 

 short, without flagella, and completely concealed from view 

 beneath the second joints. The several genera of Gecarcinidaa 

 divided into two groups or subfamilies according as they 

 have the exopodites of their outer foot-jaws provided with a 

 flagellum, and applied to the external margin of the second 

 and third joints so as to be externally visible, or have them 

 short and rudimentary, without flagella, and concealed beneath 

 the second joint, will then be distributed as follows : — 



Subfam. I. UCAIN.E. 



Genus 1. Uca, Leach. 



2. Gecarcinuctjs, M.-Edw. 



3. Cardisoma, Latr. 



Subfam. II. GECARCINIM1. 



Genus 1. Gecarcinus, Latr. 



2. Pelocarcinus, M.-Edw. 



3. Htl^iocarcinus, Wood-Mason. 



A careful study of all the numerous figures and descriptions 

 of species of Gecarcinidge, and, in the cases of the genera 

 Cardisoma and Gecarcinucus, of actual specimens, has con- 

 vinced me that the Gecarcininm further agree with one another 



