STATE GEOLOGIST. 45 



larged spines near the angle of the shell and the form of the an- 

 tennules as well as some other points, show a transition through 

 that species toward the present genus. The general form is quad- 

 rate Avith the lower posterior margin sinuate; in young specimens 

 the shell is nearly a perfect rectangle. The upper margin is pro- 

 duced 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 while the beak propt r is between the ante- 

 rior margins of the valves; the pigment fleck is rather large and 

 variously shaped. The fornices are larg^er than in Scapholeberis- 

 and extend to the front of the head over the eyes; the antennules 

 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 ca^ca. 



The members of this genus are among the most abundant and 

 conspicuous of the family and are more persistent during the 

 changing seasons than any other form. S. vetulus, the common- 

 est 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'anasta- 

 mosing lines which, in some species, show clearly their derivation 

 from a rather fine hexagonal marking. 



The sexual periods, wheu males are produced, occur in autumrt 

 and spring. The males have few distinguishing characteristics^ 

 the form being that of the young female. 



The opening of the vasa differentia is back of the anus, hence 

 these ducts cross the course of the intestine. They have ejacula- 

 tory muscles about the lower part. The smaller species are fre- 

 quently deeply colored with pink, purple and brown fatty deposits 

 and the markings are more conspicuous than in the American 

 Eurycercus, which is itself often brightly spotted with blue or pur- 

 ple. The aspect in the water is between that of Eurycercus and 

 Daphnia. 



The first mention made of any member of this genus in America is 

 Say's description, repeated in Dekay's Crustacea of New York, of 

 Daphnia angulata. This description which follows is quite sufii- 



