12 [JanuaRy, 
datr of legs ts the stouter, is of great weight, much greater than previous authors 
have recognised. In the Brachyura, the anterior pair is uniformly the strong 
pair ; and this uniformity through so extensive a group shows that the variations 
from it must be of importance in classification. This peculiarity of the Brachyura 
is a consequence of the concentration of force in the cephalic or anterior portion 
of the cephalothorax; and the diffusion of this force posteriorly, which in differ- 
ent degrees marks the Macroura, is especially exhibited in the legs. It is there- 
fore of no little interest to observe whether the first or the second pair is the 
larger, or whether the degradation is still greater and the 3d pair is chelate like 
the 2d and even stouter, as in the Penzi. By regarding this character we are 
led to place Hippolyte and Rhyncocinetes with Alpheus, instead of with Palemon; 
also Hymenocera and Pontonia with Palemon, instead of with Alpheus; Pasi- 
pha in a distinct group from the Penxi, &c. Moreover, the Penzinea, viewed 
in this light and stripped of some unrelated genera, make a natural group, for 
they are characterized by having the third pair of legs dike the second, instead of 
like the fourth. In the lowest forms among the Penzinea, there-are 7o chelate 
or didactyle legs, and the species approach the Schizopods. 
In the preceding paragraphs we have but hinted at some of the more prominent 
principles involved in the classification of the Macroura here presented, a fuller 
exposition of which will be given in another place. Below is a synopsis 
of the arrangement thus arrived at, and following this synopsis, are our descrip- 
tions of new species. 
Synopsis Familiarum Crustaceorum Macrourorum. 
I. THALASSINIDEA, vel MACROURA PAGURO-SQUILLIDICA. 
Carapax sutura transversa notatus, posticeque sepe suturis duabus longi- 
tudinalibus.. Abdomen sepius multo elongatum. Antenne externe squama 
basali sive nulla sive parvula instructe. Pedes 2 antici prorsum projecti; 6 pos- 
tici habitu raro consimiles. Species fossores. 
LegioI. THALASSINIDEA EUBRANCHIATA. 
Branchiis thoracicis instructa tantum. 
Fam. 1. Gersipm.—Maxillipedes externi pediformes. Appendices caudales et 
aliz abdominales late. 
Fam. 2. Catnianassipm.—Maxillipedes externi operculiformes. Appendices 
caudales late. 
Fam. 3. Tuatasstnip&.—Maxillipedes externi pediformes. Appendices cau- 
dales lineares. 
Legio ll. THALASSINIDEA ANOMOBRANCHIATA. 
II. ASTACIDEA vel MACROURA SUPERIORA. 
Carapax sutura transversa sepius notatus, lateribus anterioribus epistomate 
connatis. Antenne externe squama basalisive nulla sive parva instructe. Ab- 
domen sat breve vel mediocre. Branchiz penicillate. Pedes 2 antici oblique 
projecti; 6 postici directione consimiles. 
1. Antenne externe squama basalt non instructe. Pedes antici monodactylt. 
Fam. 1. Scyntartpm.—Carapax valde depressus, marginibus lateralibus sat 
tenuibus, carapace lateraliter subito inflexo. Antenne externe laminate, 
breves. Sternum trigonum. 
Fam. 2. Parinuripm.-—Carapax subcylindricus, lateraliter late rotundatus. 
Antenne externe basi subcylindrice, longe. Sternum trigonum. 
2. Antenne externe squamda basali instructe. Pedes antici didactylr. 
Fam. 3. ERryonipm.—Carapax non oblongus, depressus, lateribus subito inflexis, 
abdomine multo angustiore. 
Fam. 4. Asracipm.—Carapax oblongus, subcylindricus, abdomine parce angus- 
tiore. Sternum angustum. 
