68 



TWELFTH ANNUAL REPORT. 



posteriorly tijl 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 America; one sixth being 

 peculiar to it. 



I. Genus Macrothrix, Baird. 



Body oval, pointed behind; head broad; antenna3 of first pair 

 long, nearly straight, beset with spines, olfactory threads terminal ; 

 swimming antennae large and powerful, propelling the animal 

 by bounds; three-jointed ramus with a greatly elongated seta which 

 is thorned and jointed; labrura 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 caeca in front or behind. 



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

 0. F. Mueller whose Daphnia curvirostris is usually referred to 

 Macrothrix laticornis. 



The name Echinisca 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 M. laticornis was described and figured. ^ Nearly two years 

 later the male of M. rosea was described from Wisconsin by E. A> 

 Birge. Descriptions of the male of Lathonura are also given in 

 both the above mentioned sources. 



Sp. 1. Macrothrix laticornis, Jurine. 

 (Plate C. Figs. 7, 8 and 9.) 



Daphnia eurviro8tris(,f), jviueller. 

 Monocuhis laticornis, jxjkine. 

 Lynceus laticornis, desmabest. 



1 Gruber und Weismann, Ueber einige neue oder nnvoUkommen gekannte Daph 

 niden. Freiburg. 



