648 REPORT OF COMMISSIONER OF FISH AND FISHERIES. 



aud more minute comparison has, however, revealed some differences, 

 which are apparently constant. In the form and proportions of the seg- 

 ments of the thorax and abdomen, in the size and form of the eyes, in 

 the minute details of antennulte, antenuje, and mouth ax)pendages, I can 

 detect no differences by which it would be possible to distinguish speci- 

 mens taken in Lake Superior from those sent from Lake Wetter, or from 

 the beautiful figures of the Scandinavian species given by Sars.* In 

 the first pair of legs, however, the propodus in the American species is 

 proportionately a very little shorter than in the European, and the pal- 

 mary margin is less oblique — that is, it is not so nearly parallel with the 

 posterior margin ; the posterior margin is somewhat shorter, and fur- 

 nished with fewer hairs ; and there are usually two small and slender 

 spines on the palmary margin near the tip of the closed dactylus, while 

 in the European species there are no real spines upon the palmary mar- 

 gin, but only slender setiform hairs. In both the European and Amer- 

 ican species, there is a very thin and narrow lamellar edge, extending 

 nearly the whole length of the palmary margin. The dactylus is appar- 

 ently a little longer and more slender in the European species. The 

 obliquity of the palmary margin, and its armature near the posterior 

 angle, seem to be always characteristic of the American species. In 

 young specimens, however, there is often but one spine, while in larger 

 ones there are often three. In the third and fourth pairs of legs of the 

 American species, the dactylus is usually armed on the inside, a little way 

 from the tip, with two setiform hairs, while in the European species there 

 is only one. Some young specimens of the American species, however, 

 agree with the European in having but one hair upon the dactylus, while 

 large ones often have three, and in the full-grown male from Lake Mich- 

 igan, mentioned farther on, there are even four. 



The most remarkable differences are in the peculiar, elongated, papilli- 

 form appendages upon the sternal portion of the thoracic segments. 

 In the European species. Dr. G. O. Sars describes and figures an 

 elongated and slender process depending from the middle of the ster- 

 num of several of the thoracic segments ; and in the single specimen 

 which I have examined, there are three of these processes, one each on 

 the second, third, and fourth segments. Dr. Sars, who has studied the 

 living animals very carefully, does not suggest what may be the use of 

 the appendages, or whether they ever vary in number or position in 

 different specimens. In specimens from Lake Superior, there are usually 

 seven of these appendages, one upon the second and two each upon the 

 third, fourth, and fifth segments. In form aud size, these appendages 

 do not differ, except that in alcoholic specimens they seem to be a little 

 longer in the American species. In some specimens of the American 

 species, the appendage upon the second segment is wholly wanting, and 

 in two specimens examined carefully there was only a single median 



* Histoire Naturelle des Crustaces d'Eau Douce de Norvege, p. 82, pi. 7, figs. 10-25 ; 

 pi. 8, figs. 1-5, 1867. 



