4 Mr. C. S. Bate on the Morphology of some Amphipoda 



distal angle produced nearly to the extremity of the propodos ; 

 in the third, it does not reach to half that length ; in each 

 the propodos is anteriorly hollowed and distally produced, so 

 that the dactylos impinges between two processes, and terminates 

 in a distinctly curved unguis. The fifth pair of pereiopoda, both 

 in the adult and in the larva, are very short, appearing in the 

 full-grown animal as a perfectly formed but feeble organ, whilst 

 in the young they resemble an imperfectly developed appendage. 

 In the young, none of the pleopoda are properly developed, and 

 only two pairs are present, in an embryonic condition ; the tel- 

 son appears as broad as the previous segments ; while in the 

 adult the three posterior pairs of pleopoda are long and bira- 

 mous, the telson being small and lanceolate. 



Platyscelus, nov. gen. 



This genus agrees in every respect with Dana's genus Di- 

 thyrus, except that, after the basa in the third and fourth pairs 

 of pereiopoda, the remaining joints are developed, whereas in 

 Dithyrus they are wanting. 



Platyscelus serratus. 



Cephalon transversely ovate. Pereion broadly distended. 

 Pleon narrow. Eyes occupying the entire lateral walls of the 

 cephalon. Superior antennae short, six-jointed ; the first three, 

 or peduncle, stout ; the fourth, or first articulus of the flagellum, 

 equally stout, truncate at the extremity, where stands a bunch 

 of auditory cilia ; the last two articuli are narrow, and terminate 

 sharply. Inferior antennae short, four-jointed, the joints not 

 inflected one upon the other, but lying extended, and hidden be- 

 neath the cephalon. First pair of gnathopoda complexly sub- 

 chelate ; meros broad ; carpus broad and long, the inferior mar- 

 gin anteriorly produced, and serrated both on the anterior and 

 inferior margins ; propodos a long-ellipse, and strongly serrated 

 both on the superior and inferior margins ; dactylos sharp and 

 smooth. Second pair of gnathopoda formed upon the same 

 type as the first, but longer and not quite so broad. First two 

 pairs of pereiopoda simple, but having the posterior or flex- 

 ible margins minutely serrated. Third pair of pereiopoda short, 

 having the basos long, being as long again as broad ; anterior 

 -margin nearly straight, having the distal extremity serrated; 

 posterior margin arcuate, the internal surface concave ; the re- 

 maining joints scarcely more than half the length of the basos, 

 and having the anterior or flexible margin serrated; dactylos 

 smooth, sharp. Fourth pair of pereiopoda having the basos 

 extremely developed ; the anterior margin excavate, corresponding 

 in form with the posterior of the preceding pair ; posterior 



