642 On some Entomostraca of Lake Michigan [August, 



P. E. Miiller's supposition, to restore the balance of the body 

 and enable it to float feet downwards, the shell secretes a layer or 

 cloak of a gelatinous character and of an enormous thickness, 

 relatively to the size of the animal. Through a slit in this man- 

 tle the antennae and feet are thrust out ; but otherwise the animal 

 is completely buried in a lump of gelatine. 



Bosmina (PI. ix Fig. 17) was less abundant in my collections than 

 the otherforms mentioned, but occurred very commonly in the stom- 

 achs of My sis oculatus dredged from the deeper waters of the bay. 



The commonest Cladocera in the lake are two forms of Daph- 

 nia, remarkable for their thinness and exquisite transparency. 

 They are allied to galeata and pellucida of the old world, recently 

 reduced by Adolph Lutz to varieties of Daphnia hyalina Leydig. 

 Although our specimens do not agree strictly either with the de- 

 scriptions or the figures of those varieties extant, their differ- 

 ences probably do not pass the limits of allowable variation in 

 this excessively variable species. The head is keeled, convex m 

 dorsal outline and either rounded (pellucida) or pointed (galeata) 

 in front, the shell is compressed and reticulate, and terminates 

 posteriorly in a long, straight, dentate spine. 



An allied species, from the smaller Illinois lakes, where it is in 

 autumn by far the most abundant entomostracan, resembles Daph- 

 nia cederstromii Schodler, but differs especially in the still more 

 enormous development of the head. This is as high as the body 

 and more than two-thirds as long, deeply concave on the upper 

 border, the apex curving upwards far beyond the dorsal line of the 

 body. The head is expanded inferiorly also to such a degree that 

 the sensory hairs of the antennules fall much short of the tip of 

 the rostrum. The shell is reticulate, and its spine long and 

 straight, there is no macula nigra, and the caudal claws have a 

 row of teeth at their base. For this curious form I propose the 

 name of Daphnia rctrocurva. 



I have not found it in Lake Michigan, although 

 lakes it is mingled with both varieties of Daphnia hyalina. Even 

 in the young, before they have left the brood cavity of the mother, 

 the helmet is developed far beyond that of the adult of any of the 

 latter species. 



The female carries but one or two eggs, and the young some- 

 times attain a size more than half that of the body of the mother 

 within the shell, before they leave her protection. 



. smaller 



