121 Botany and. Zoology. 
the palpus is long and 3-jointed ; if united for some distance up, the 
palpus becomes short and finally only 2-jointed (Palzemonella ;) if united 
nearly or quite to their summits, there is no palpus.* vel 
In the arrangement of the genera into families, the fact whether the 
lst or 2d pair of legs is the stouter, is here of great weight, much 
greater than has been recognized. 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. 
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 families and sub-families thus arrived at, 
with a description of the new genera. 
Synopsis Familiarum Crustaceorum Macrourorum. 
Subtribas IL. THALASSINIDEA, vel MACROURA PAGURO- 
SQUILLIDICA. 
Carapax suturá transversá notatus, posticeque ssepe suturis duabus 
longitudinalibus. Abdomen szpius multo elongatum. Antenne ex- 
terne squamá basali sive nullá sive parvulá instructa. — Pedes 2 antici 
prorsum projecti ; 6 postici habitu raro consimiles. Species fossores. 
Legio lL THALASSINIDEA EUBRANCHIATA. 
Branchiis instructa thoracicis tantum. 
Fam. 1. Gzarp.g.— Maxillipedes externi pediformes. Appendices cau- 
dales et alie abdominales latze.—GENERA: Gebia, Leach, Axius, 
Leach, Calocaris, Bell, Laomedia, De Haan, Glaucothoe, Edw.t 
Fam. 92. CanLiANASsIDE.—Maxillipedes externi operculiformes. Ap- 
pendices caudales latee.—GzENwERA : Callianassa, Leach, 'Trypzea. D. 
Fam. 3. THALAssiNiDz.—Maxillipedes externi pediformes. Appen- 
dices caudales lineares.—GNvs : Thalassina, Latr. 
Legio IL THALASSINIDEA ANOMOBRANCHIATA. - 
GENERA :—Callianidea, Edw., Callisea, ( Guerin,) D. | 
Subtribus II. ASTACIDEA vel MACROURA SUPERIORA. 
Carapax suturá transversá szepius notatus, lateribus anterioribus epis- 
tomate connatis. Antennz externae squamá basali sive nullá sive 
* ]n our genus Palemonella, the palpus of the mandible is 2-jointed, and in An- 
chistia, which is closely like Palemon in habit in some of its species, thére is no 
palpus, as in the Pontoniz ; and thus the transition to the Pontonis fromi Palemon 
is exceedingly gradual. Harpilius and CEdipus (Pontonia of authors) fill up the in- 
terval between Anchistia and the true Pontonim. They are all similar in having the 
2d pair of legs largest, and in other prominent characteristics. 
1 The genera of living species only are mentioned. 
