306 Birge — Notes on Cladocera. 



Pleuroxus nanus, Baird. 



This minute species was found in Green lake, at Minocqua 

 and New Lisbon, Wis., and at Isle Royale, It has been re- 

 ported from Minnesota by Herrick. 



Pleuroxus hastatus, Sars. 

 Plate XIII, Fig. 11. 



A form which, on the whole, seems to me to belong to this, 

 species was found in a marshy pool near New Lisbon, Wis. In 

 general shape and proportions it resembles P. gracilis, Hud.,, 

 and thus is much longer than high. In this respect it differs 

 greatly from P. E. Mueller's figure of P. hastatus, and in less 

 degree from the figures and measurements of Hellich, Kurz 

 and Norman and Brady. Hellich is the only author who 

 gives dimensions of both height and length. His results are, 

 length, 0.55-0. 6 mm., hight, 0.32-0.35 mm. My largest specimens 

 measure 0.51 mm. by 0.22 mm. They are nearly as long as those 

 of Hellich but only about two-thirds as high. In spite of this 

 difference I consider the species the same. The posterior mar- 

 gin of the shell has the same form as that of the European 

 specimens. The same is true of the size, shape and armature of 

 the postabdomen, the length of the beak and the proportions of 

 the antennules. 



European authors differ regarding the cilia of the ventral 

 edge of the valves. Kurz (74, p. 66) places them in the middle 

 only. Hellich (77 p. 102), figures them along the entire ventral 

 margin, only slightly shorter at the posterior end. Mueller ('68 

 p. 193, P. Ill, f. 25), apparently sees them about like Hellich. 

 I find that they almost but not quite disappear at the posterior 

 end of the valves, but are much the same as in most species of 

 Pleuroxus. 



The marking of the valves in alcoholic material consists of 

 striae anteriorly and obscure reticulations behind. In some 

 specimens the posterior part of the valves seems obliquely- 

 striated as in so many of the species of this genus. 



