Q94 CRUSTACEA. 



ciliated. Yet the fourth and fifth pairs, instead of being styliform, 

 as in Amphipods, are branchial, as in Isopods. Such are the species 

 of the Serolis group. This is a divergence from the Isopod type of 

 structure of a very wide and important character; and still the 

 general habit of the species is Tsopodan. 



Prom these observations, it is obvious that there is a group of Cho- 

 ristopods which has not hitherto been recognised. Till now, they 

 have been mostly placed with the Isopods ; and it is of much interest 

 to observe, that nearly all of the larger groups in Edwards's system, 

 are divided by him into two parts, one part including the true Iso- 

 poda, the other, the species here classed as Anisopoda. In his more 

 recent paper on the Serolis group, he recognises the great difference 

 between the species and other Isopoda, but still continues them with 

 that tribe, referring them to its lower grades, or as the link between 

 the Isopoda and Trilobita. 



The characters of the tribes are as follows : — 



Tribus I. Isopoda. — Appendices branchiales pedes sunt abdominales, 

 foliaceae. Pedes thoracici seriei anticae antrorsum porrecti numero 

 sex. Pedes abdominales decern antici foliacei, duo postici saepe 

 styliformes. Abdomen breve. 



Tribus II. Anisopoda. — Appendices branchiales pedes sunt abdomi- 

 nales, foliaceae. Pedes thoracici seriei anticae antrorsum porrecti 

 numero octo (ac in Amphipodis) . Pedes abdominales decern antici 

 foliacei, interdum subnatatorii, duo postici saepe styliformes. Abdo- 

 men breve vel breviusculum. 



Tribus III. Amphipoda. — Appendices branchiales pedibus thoracis 

 appendiculatae. Pedes thoracici seriei anticae antrorsum porrecti 

 numero octo. Pedes abdominales sex antici elongati, natatorii, reli- 

 qui sex plus minusve styliformes. Abdomen elongatum, raro obso- 

 letum. 



It is obvious that the Anisopoda constitute a type intermediate 

 between the Isopoda and Amphipoda ; but not so apparent, whether 

 the Amphipoda or Isopoda should rank first in order. 



The position of the branchiae in the Amphipoda, as appendages to 



