12 [January, 



dair of legs is 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 

 i 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 Penaei. By regarding this character we are 

 led to place Hippolyte and Rhyncocinetes with Alpheus, instead of with Palaemon ; 

 also Hymenocera and Pontonia with Palasmon, instead of with Alpheus; Pasi- 

 phaea in a distinct group from the Penaei, &c. Moreover, the Penaeinea, 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 liJce the second, instead of 

 like the fourth. In the lowest forms among the Penaeinea, there are no 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 Familiar2im CrustaceorJirti Macrourorum. 



I. THALASSINIDEA, vel MACROURA PAGURO-SQUILLIDICA. 



Carapax sutura transversa notatus, posticeque saepe suturis duabus longi- 

 tudinalibus. Abdomen saepius multo elongatum. Antennae externae squama 

 basali sive nulla sive parvula instructae. Pedes 2 antici prorsum projecti ; 6 pos- 

 tici habitu raro consimiles. Species fossores. 



Legio I. THALASSINIDEA EUBRANCHIATA. 



Branthiis thoracicis instructa tantum. 



Fam. 1. Gebid^. Maxillipedes externi pediformes. Appendices caudales et 

 aliae abdominales latae. 



Fam. 2. CALLiANASsiDyE. Maxillipedes externi operculiformes. Appendices 

 caudales latae. 



Fam. 3. Thalassinid^. Maxillipedes externi pediformes. Appendices cau- 

 dales lineares. 



Legio II. THALASSINIDEA ANOMOBRANCHIATA. 

 IL ASTACIDEA vel MACROURA SUPERIORA. 



Carapax sutura transversa saspius notatus, lateribus anterioribus epistomate 

 connatis. Antennae externae squama basali sive nulla sive parva instructae. Ab- 

 domen sat breve vel mediocre. Branchiae penicillatae. Pedes 2 antici oblique 

 projecti; 6 postici directione consimiles. 



1. Antenna externce squama basali non instmctcB, Pedes aiitici monodactyli , 

 Fam. 1. ScYLLARiB^E. Carapax valde depressus, marginibus lateralibus sat 

 tenuibus, carapace lateraliter subito inflexo. Antennae externae laminatae, 

 breves. Sternum trigonum. 

 Fam. 2. PALiNURiOiE. Carapax subcylindricus, lateraliter late rotundatus. 

 Antennae externae basi subcylindricac, longae. Sternum trigonum. 

 2. Antennae externce squavid basali instrncta-. Pedes antici didactyh. 

 Fam. 3. Eryonid^. Carapax non oblongus, depressus, lateribus subito inflexis, 



abdomine multo angustiore. 

 Fam. 4. Astacid.'e. Carapax oblongus, subcylindricus, abdomine parce angus- 

 tiore. Sternum angustum. 



