24 CONNECTICUT GEOL. AND NAT. HIST. SURVEY. 



[Bull. 



The joints are all movable in a fore and aft direction, except 

 the first or coxal plate, which is attached to the thorax in such a 

 way as to be capable of a slight lateral movement only, and from 

 the median side of which the succeeding joint of the pereiopod 

 arises. The ischium is always short, scarcely longer than broad 

 except in the second gnathopods of the Lysianassidse. The 

 succeeding joints vary greatly in different species in both their 

 shape and size ; the terminal one, however, is usually claw-like. 



The coxal plates vary greatly in depth and length both among 

 themselves and in different species. They are very deep, for 

 example in Stenothoe, Metopa, Stegocephalus, and the Lysianas- 

 sidffi, while in the Corophiida they are small and low, not forming 

 a continuous series of overlapping plates as is more common. 

 There is a gradual increase in size from the first to the fourth, 

 which is generally the largest. The fourth may be excavated at 

 the upper posterior angle to accommodate the coxal plate of the 

 fifth pereiopod, but in the following families it is not excavated : 

 Stenothoidas, Aoridas, Photidse, Amphithoidae, Jassidae, and 

 Corophiidse. The fifth, sixth, and seventh coxal plates are often 

 considerably smaller than the preceding, diminishing posteriorly, 

 and are bilobed. Among the Caprellidea the coxal plates are 

 often absent and when present are extremely minute. 



The gnathopods exhibit great differences in individual species 

 as well as in the two sexes and at different ages. The first pair 

 is smaller than the second except in the genera, Ampelisca, 

 Microdeutopus, Lemhos, Unciola, and Leptocheirus, where the 

 condition is reversed. The propodus is usually subchelate ; 

 that is, the posterior and distal margins form a palm against 

 which the dactyl strikes when flexed, so that the appendage is 

 adapted for grasping. The angle made by the posterior margin 

 and the palm is known as the prehensile angle and may be armed 

 with one or more stout spines. The dactyl may have its concave 

 margin modified by serrations or processes. In a few species 

 the dactyl does not close against the propodus, in which event 

 the gnathopod is known as simple. This is more frequently the 

 case with the first pair than with the second. Thus in Lysianop- 

 sis alba, Leptocherius pinguis, and the females of Talorchestia, 

 the first gnathopod is simple, and in Ampelisca spinipes, the 

 second is simple. Besides the simple and subchelate forms of 



