APPENDIX. one 
& 
On the Classification of the Crustacea Choristopoda or Tetra- 
decapoda ; by James D. Dana. . 
Tue term Choristopoda, applied to the Tetradecapods, alludes to the 
subdivision of the thorax into segments, each devoted to a separate pair 
e this is a prominent peculiarity of the species, distinguishing 
them from all the Podophthalmia, and with rare exceptions from the 
Entomostraca. 
1. The three posterior pairs of thoracic legs thrown backward and more 
or less obliquely outward, and constituting 0 
irs are throw 
series; this arrangement may be represented by the figures 4:3, 
(or 24+2:3, as the four pairs of the first series are often in two 
sets of two pairs each). 
2. The branchial appendages thoracic. 
e abdominal members in two sets, the three anterior pairs sub- 
natatory, the three posterior styliform—an arrangement repre- 
sented by the figures 5: 3. 
~ The true Isopopa, on the contrary, have— a 
1, The four posterior pairs of thoracic | gs in the backward series, 
and three anterior pairs in the forward series—3 : 4. 
al. pee . 
8. The abdominal members in two sets, the 5 anterior pairs branchial (the _ 
first sometimes operculiform), and only the last styliform—o : 1. 
_ These are two distinct types of sir 
and any act dee partake of these peculiarilies are inter- 
- Szcoxp Series, Vol. XIV, No. 41.—Sept., 1852. 38 
