FEESH-WATER CRUSTACEA OF THE UNITED STATES. 645 



auce, as long as we recognize the characters which distinguish them ; 

 bnt, as we know no truly intermediate forms, it is perhaps best for the 

 present to regard it as a species. 



In regard to the distribution and habits of this species in Europe, I 

 transhite the following remarks from Dr. Sars's great work. He says, 

 " I have found it in Norway only in Lake Mjosen, the largest of our 

 lakes. There, however, it is found in very great quantities, from shal- 

 low water (3 to 6 fathoms) to very great depths, (200 fathoms.) In 

 Sweden, it seems to be much more widely diffused. Besides the two 

 largest lakes of that country, Wener and Wetter, where it was first dis- 

 covered, it has since been found iu eight other Swedish lakes as well 

 as iu the Gulf of Bothnia. It has also recently been fonnd by M. Malm- 

 gren iu Lake Ladoga, the largest lake in Europe, as well as iu many 

 of the lakes of Finland. In habits, it seems to resemble the marine spe- 

 cies. Like them, it generally liv^es collected together in great masses or 

 in bands. It seems to prefer places where the bottom presents quite 

 rapid inclinations; in such places, it is frequently found in great bands, 

 swimming along the borders of these acclivities iu the calm and elegant 

 manner peculiar to the species of Mysis, making a digression only here 

 and there to avoid some object which it fears. Its principal food seems 

 to be composed of Entomostraca, with which these waters swarm. In 

 the stomach of an individual, which I examined for the purpose, I dis- 

 covered the remains of two or three species of Cyclops, a CantJiocam2)tus, 

 a Bosmina, a Daphnia, and a Cyiwis.''' 



AMPHIPODA. 



Family Orchestid^. 



Hyalella, (jen. nov. 



First pair of maxillie with rudimentary, very short, and uniarticu- 

 late palpi. Palpus of the maxillipeds composed of five segments; the 

 terminal segment being slender and styliform, and the penultimate 

 broad. Autennulte, antenmie, and thoracic legs much as in Hyale. Tel- 

 sou short, stout, and entire. 



This genus seems to be closely allied to Hyale, but differs from it and 

 from the rest of the Orcliestidce in the palpus of maxillipeds, which has 

 five instead of four segments, showing in this respect a remarkable 

 approach toward the gammaroid group of Ampliipoda. From Hyale, it 

 differs also iu the telson. 



Hyalella dentata, sp. noi-. (Plate II, fig. 8, male, lateral view; fig. 



9, female, lateral view; tig. 10, details.) 



Body slightly compressed. First and second segments of the abdo- 

 men with the dorsal margin produced posteriorly into a well-marked 

 spiuiform tooth. Eyes nearly rouud, about equal in diameter to the 

 thickness of the proximal segment of the peduncle of the antennula. 



