FEESH-WATER CRUSTACEA OF THE UNITED STATES. 645 



ance, as long as we recognize tlie characters which distinguish them ; 

 but, 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 

 translate 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 ftithoms.) 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 beea found in eight other Swedish lakes as well 

 as in the Gulf of Bothnia. It has also recently been found by M. Malm- 

 gren in Lake Ladoga, the largest lake in Europe, as well as in 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 in 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 JEntomostraca, 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 Canthocamptus, 

 a Bosmina, a DapJmia, and a Cypris.'^ 



AMPHIPODA. 



Family Orchestid^. 



Hyalella, gen. nov. 



First pair of maxilloe with rudimentary, very short, and uuiarticu- 

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

 terminal segment being slender and styliform, and the penultimate 

 broad. Antennulte, antennie, and thoracic legs much as in Hyale. Tel- 

 son short, stout, and entire. 



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

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

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

 approach toward the gammaroid group of Amiihipoda. From HyaJe, it 

 differs also in the telson. 



Hyalella dentata, sp. not\ (Plate II, fig. 8, male, lateral viewj fig. 



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



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

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

 spiniform tooth. Eyes nearly round, about equal in diameter to the 

 thickness of the proximal segment of the peduncle of the antenuula. 



