No. 26.] ARTHROSTRACA OF CONNECTICUT. 187 



The first maxillse lack a palp in all the Isopoda except the 

 Tanaioidea in which a backwardly directed one is present. The 

 Tanaidse and yEgidse exhibit a single lobe in the first maxillae 

 whereas typically the first maxillse consist of two unequal lobes, 

 the inner of which is the smaller. The second maxillse are typi- 

 cally tri-lobed at the apex but in the Oniscoidea they are laminar 

 with only a slight indication of division into lobes; in the ^gidse 

 there are only two lobes and in the Tanaidse the second maxillse 

 are quite rudimentary. 



The maxillipeds bear a palp of a varying number of joints 

 and have a single expansion corresponding to the inner plates of 

 the Amphipoda. In the Tanaoidea the epipod extends within the 

 branchial cavity beneath the lateral margins of the carapace and 

 serves as a bailer like the scaphognathite of the Decapoda. In 

 the yEgidse the epipod becomes fused with the basal joint. 



In the AnLhuridse the mouth parts are suctorial rather than 

 masticatory in function. 



The thorax is always distinctly segmented, and in the New 

 England forms, with the exception of the Tanaioidea, is made 

 up of seven free joints. In the single group mentioned the first 

 segment is fused with the head. 



The legs are normally fourteen in number, and made up of the 

 same number of joints as in the Amphipoda. The most important 

 distinction from the legs of the latter group is in the coxal plates 

 which are usually not separated from their respective segments 

 and are never as movable as in the Amphipoda. In no case is the 

 first coxal plate separated from its segment. In the Tanaidse 

 the coxal plates, or epimera, are quite small, while in the Onis- 

 coidea they are greatly expanded. 



It is only in the Oniscoidea that the legs are sufficiently 

 similar in structure to warrant the name of the whole group — 

 Isopoda. The first pair are frequently quite different from the 

 succeeding ones. In the Tanaidse the first pair are chelate, and 

 in many of the other groups the first one or two pairs are sub- 

 chelate. 



In the females oostegites are attached to four pairs of append- 

 ages, generally to the first four but occasionally to appendages 

 two to five inclusive. The Cymothoidse are exceptions, bearing 

 five pairs of oostegites on the first five pairs of appendages and 



