18 J. D. Dana on the Classification of Crustacea. 



the preceding, producing lower grades of species. They are all 

 marks of a relaxation of the centralization. 



Fourth. An enlargement or widening of the sternum and ab- 

 domen. 



Fifth. The abdomen becoming somewhat relaxed from the 

 venter instead of remaining close- appressed to it. 



Sixth. The vulvae becoming more remote from one another, 

 being situated in the bases of the third pair of legs, instead of 

 the sternum. 



Seventh. The inner antennae losing their fossettes, and being 

 constantly exsert. 



Eighth. The branchiae being more than nine in number on 

 either side. 



The first of these peculiarities distinguishes many of the 

 Grapsoids, as well as lower species. The second is observed in 

 the Corystoids, and is an additional mark of their inferior grade. 

 The third occurs in Dromia and allied. The fourth, in Latreillia. 

 The fifth, in Dromia. Dromia and Latreillia have the posterior 

 legs abbreviated, and in Dromia, this evidence of degradation is 

 still stronger, in that the fourth as well as fifth pair is short and 

 dorsal. 



The last three characteristics, above mentioned, mark a tran- 

 sition towards the Macroural type, and the genera of this kind 

 belong with the Anomoura. This transition is seen further in — 



Ninth. The eyes being without fossettes. 



Tenth. The second pair of antennae becoming exterior to the 

 eyes. 



Eleventh. The outer maxillipeds more enlarged and subpe- 

 diform. 



Twelfth. The abdomen more lax and furnished with a pair of 

 caudal appendages. 



Thirteenth. The abdomen more elongated, and hardly inflexed. 



These several changes exhibit a continuation of the process of " 

 relaxation in the central forces. There is thereby an enlarge- 

 ment of the antennae, and their more remote position at the an- 

 terior extremity of the animal ; and also, an enlargement of the 

 posterior or abdominal parts of the animal, and a development 

 of appendages in the posterior direction. These marks of de- 

 gradation, excepting the thirteenth, are found in the Hippa and 

 Porcellana groups, and the thirteenth in the Paguridea. At the 

 same time that these Macroural characteristics appear, the body 

 becomes elongated. The species all bear a stamp of imperfection 

 in the abbreviated posterior legs, as explained above, as well as 

 in the other points alluded to. The subordination of the nine 

 anterior annuli to cephalic functions, which is so striking in the 

 Maioids, has become less and less complete, and the organs less 

 perfect ; moreover, the habits of the animals are more sluggish, 



