No. 26.] ARTHROSTRACA OF CONNECTICUT. 23 



partial; but in all other Amphipoda the plates are separate and 

 palps are present. The plates are most highly developed in the 

 Lysianassidae. They are obsolete in Stenothoe. The inner plates 

 usually bear numerous setae on their inner margins and may bear 

 tooth-like spines apically which are of use in mastication. The 

 outer plate is usually furnished with strong spines and setae on 

 the inner and distal margins. In Amphithoe and Grubia the 

 sickle-shaped spines on the inner margin gradually increase in 

 length toward the apex and become simple slender setae. The 

 first joint of the palp is always short; the second joint is usually 

 cylindrical, but in Orchestia it bears a distal laminar prolongation 

 resembling the inner and outer plates; the third joint is club- 

 shaped or ovoid; and the fourth joint is somewhat conical and 

 may be armed with a stout spine distally. In the genera 

 Haustorius, Orchestia, and Talorchestia the palp is triarticulate, 

 but in all other genera, when present, it is four-jointed. 



Gnathopods and Pereiopods. 



In all the Amphipoda there are seven pairs of thoracic append- 

 ages, except in the Caprellidea, in the New England species of 

 which all traces of limbs of the third and fourth thoracic seg- 

 ments are wanting. The first two pairs differ greatly from the 

 others and are generally referred to as gnathopods ; so that 

 the third thoracic limb becomes the first pereiopod. Each thoracic 

 appendage is made up of seven joints which, reckoning from the 

 proximal one, are designated usually as the coxal plate or coxa, 

 basal joint, or .basus, ischium, merus, carpus, propodus, and 

 dactyl. In Haustorius the dactyls are wanting in the case of the 

 pereiopods, although present in the gnathopods. Batea is unique 

 in that the first gnathopod is reduced to a mere rudiment repre- 

 senting the coxal plate and basal joint. 



The first two pairs of pereiopods, like the gnathopods, are 

 usually held flexed forward quite strongly at the joint between 

 the ischium and merus, and the dactyls are directed backwards. 

 They are usually of similar form and size and have slender basal 

 joints. The third, fourth, and fifth pereiopods are more or less 

 flexed backward at the joint between the ischium and merus and 

 the dactyls are directed forwards. The basal joints of the last 

 three legs are typically expanded to form flat plates. 



