096 CRUSTACEA. 



above conclusion, inasmuch as the abdomen in these species is nearly 

 or quite obsolete. These are, however, aberrant forms, bearing 

 a resemblance in essential points of structure to the Amphipoda. 

 They have the long antennae of the Amphipods; and whenever the 

 abdomen is partly developed, as in some of the species, it has rudi- 

 ments of the same members that are observed in that tribe. 



These species — the Caprellidse — are also peculiar in often wanting 

 the third and fourth pairs of legs, so that the cephalothorax consists 

 of two distinct divisions. This tendency to abortion in the legs at 

 the middle of the thorax, is also presented by some of the Macroura, 

 as in certain Crangonidae. The posterior part of the thorax in such 

 cases appears to be in some sense separate from the anterior in its 

 developments, as if pertaining to a different centre. 



Tribe I. ISOPODA. 



The tribe Isopoda, as here adopted, corresponds to that of other 

 authors so called, except that the groups named by Milne Edwards 

 "Idoteides Arpenteurs," "Asellotes Heteropodes," " Praniziens," 

 " Bopyriens," and the Serolis group, are excluded, as they are properly 

 Anisopoda. There are three subtribes. 



I. IdoTuEidea. — Appendices abdominales duae posticse bene operculi- 

 formes, appendices alias optime tegentes. 



II. Oniscoidea. — Appendices abdominales duae posticse styliformes et 

 non operculiformes, fere terminales, raro obsolete. 



III. Cymothoidea. — Appendices abdominales duae posticse lamellatse, 

 apud abdominis latera dispositae. 



