ENTOMOSTRACA OF MINNESOTA. 



211 



The males have the openiDg of the vasa deferentia in front of the 

 claws, which may be absent; the antennules are also modified, being 

 longer and curved. In Lathonura the abdomen is elongated posteriorly 

 till it begins to suggest a transition to Polyphemus. 



The known genera and their distribution is as indicated below. 

 Half of the known species are found in A.merica, one-fourth being 

 peculiar to it. 



OENUS MICROTHRIX Biird. 



Body oval, pointed behind; head broad; antennae of first pair long, 

 nearly straight, beset with spines, olfactory threads terminal; swim- 

 ming antennae large and powerful, propelling the animal by bounds; 

 three jointed ramus with a greatly elongated setae which is thorned 

 and jointed; labrum with the basal joint enlarged, resembling that of 

 Lynceids; first foot with a hook in both sexes; last foot with a long 

 process (respiratory body); abdomen short; claws short; caudal stylets 

 often with a bush of hairs at tip. The intestine is straight and without 

 cieca in front or behind. 



The first one to observe a member of this genus, apparently, was 

 O. F. Miiller whose D iphnia caroirosfris is usually referred to Macro- 

 thrix laticornis. 



The name Echbiisca was proposed by Lievin, but Macrothrix was 



applied by Baird in 1843. Four species are known, three of which 



occur in America, and without doubt the fourth will ultimately be 



found. No males of this genus were known till 1877 when the male 



of Jf. laticornis was described and figured (Grube and Weismann '80.) 



J^early two years later the male of M. rosea was described from Wis- 



coDsin by E. A. Birge. Descriptions of the vn-aAe of Lathonura are also 



given in both the above mentioned sources. 

 14 



