Prof. J. D. Dana on the Classification of Mammals. 209 



transfer of thoracic members to the cephalic series, and the re- 

 verse. There is a third law which should be mentioned to ex- 

 plain the relations of the Entomostracans to the other orders, 

 namely, (3) that a decline in grade, after the laxness and elonga- 

 tion of the anterior and posterior extremities have reached their 

 limit, is further exhibited by a degradation of the body, and 

 especially of its extremities. 



In the step down from the Decapods to the Tetradecapods, 

 there is an illustration of this principle in the eyes of the latter 

 being imbedded in the head instead of being pedicellate. In the 

 Entomostracans (1) the elongated abdomen is destitute of all 

 but one or two of the normal pairs of members, not through a 

 system of abbreviation, as exhibited in crabs, but a system of 

 degradation ; and in some species all the normal members are 

 wanting, and even the abdomen itself is nearly obsolete. Again, 

 (2) the two posterior pairs of thoracic legs are wanting in the 

 species, and sometimes more than two pairs. Again, (3) at the 

 anterior extremity, one pair of antennae is often obsolete, and 

 sometimes the second pair nearly or even quite so. TheLimuIus, 

 though so large an animal, has the abdomen reduced to a 

 straight spine, and the antennae to a small pair of piucer legs, 

 while all the mouth-organs are true legs — the whole structure 

 indicating the extreme of degradation. 



In the order of Decapods having nine as the normal number 

 of pairs of cephalic organs, the species of the highest group have 

 these organs compacted within the least space consistent with the 

 structure of the type ; in those a grade lower, the posterior pair 

 is a little more remote from the others, and begins to be some- 

 what pediform ; a grade lower, this pair is really pediform, or 

 nearly like the other feet ; and still lower, two or three pairs are 

 pediform. Still lower in the series of Decapods (the Schizopods), 

 there are examples under the principle of degradation above ex- 

 plained — (1) in the absence of two or three pairs of the posterior 

 thoracic appendages, (2) in the absence or obsolescence of the 

 abdominal appendages, (3) in the Schizopod character of the 

 feet. These Decapods, thus degraded, approximate to the Ento- 

 mostracans, although true Decapods in type of structure. Thus 

 the principle is exemplified within the limits of a single order, 

 as well as in the range of orders. 



This connexion of cephalization with rise of rank is also illus- 

 trated abundantly in embryonic development. It is one of the 

 fundamental principles in living nature*. 



* In his ' Manual of Geologj',' just published, the writer, speaking of the 

 ancient Ganoids, has preferred to use the term vertebrated tails rather than 



