ENTOMOSTEACA OF MINNESOTA. 177 



S. cornuta from America is one of the most important supports of the 

 specific independence of the two forms. All the species delight in 

 disporting themselves near the surface in sunny weather. 



GENUS SIMOCEPHALUS. 



Although a very well circumscribed group, this genus passes into 

 the next rather directly by means of S. macrothroides. The connection 

 on the other hand seems to be by the way of Scapholeberis, though 

 there is a rather broad separation between even Scaylwleberis angulaia 

 and any known Simocephalus. The enlarged spines near the angle 

 of the shell and the form of the antennules as well as some other 

 points, show a transition through that species toward the present 

 genus. The general form is quadrate with the lower posterior 

 margin sinuate; in young specimens the shell is nearly a perfect rect- 

 angle. The upper margin is produced more or less at the point of 

 union with the free posterior margin, and the shell is either arched 

 or very abruptly angled above the prominence in old females. The 

 head is produced into a projection at the eye, Avhile the beak proper 

 is between the anterior margins of the valves; the pigment fleck is 

 rather large and variously shaped. The fornices are larger than in 

 ScapJwleberis and extend to the front of the head over the eyes; the an- 

 tennules have a lateral flagellum which is large and lance-shaped. 

 The post-abdomen varies very little in shape; it is truncate and exca- 

 vated below and very broad. The anal teeth are few, large, curved, 

 pectinate; the claws are straightish and pectinate or spined; the 

 labrum is shaped as in Daphnia; the anterior part of the stomach has 

 the usual coeca. 



The members of this genus are among the most abundant and con- 

 spicuous of the family and are more persistent during the changing 

 seasons than any other form. *8'. vetulus, the commonest species, stands 

 in the centre of the genus, while two extremes are expressed by the 

 other members of the group. 



The winter or sexually produced eggs are lodged in an ephippium 

 or saddle-like modification of the shell, which is finely reticulate; while 

 the shell is usually marked by fine anastamosing lines which, in some 

 species, show clearly their derivation from a rather fine hexagonal 

 marking. 



The sexual periods, when males are produced, occur in autumn and 

 spring. The males have few distinguishing characteristics, the form 

 being that of the young female. 



The opening of the vasa deferentia is back of the anus, hence these 

 ducts cross the course of the intestine. They have ejaculatory muscles 



