ENTOMOSTEACA OF MINNESOTA. 181 



ciently like Simoce'phalus. The lower angle of the shell is not armed 

 with the peculiar curved spines as in all the other species. This species 

 becomes over one-tenth inch long. In such old individuals the spine 

 is nearly midway of the height. 



One could wish a trifle closer link to ScapJiolcberis than that furnished 

 by S. angulata; but, on the whole, the position of this genus can not 

 well be called in question. America has four species out of the six 

 known and but one of these certainly identical with the European, 

 though others are probably closely related. 



GENUS DAPHNIA. 



Long considered the type of the family, this genus is most frequently 

 seen, or, at least, is more conspicuous than any other group. It has 

 already been pointed out that the forms here united are the extreme 

 development of a diverging line. Shnocephalus is the link connecting 

 it with the typical forms of the family. As might be expected, this 

 genus presents more puzzling problems than any of the others. It con- 

 tains more peculiarities of structure and diversities of habit and de- 

 velopment than any other of the genera. Here the sexual differences are 

 most interesting. The young are hatched with a pendant appendage 

 attached to the upper posterior angle of the shell, which soon becomes 

 the rigid spine characteristic of the younger stages and males of the 

 genus. The females almost immediately after birth commence the 

 production of eggs by an asexual process. Groups of epithelial cells 

 containing four each are formed and one of the cells of each group 

 develops at the expense of the others, forming the egg. Many such 

 eggs are laid simultaneously and deposited in the cavity between the 

 shell and the dorsal part of the animal. The eggs are prevented from 

 escaping by means of three long processes, of which the first is much, 

 the larger and curves forward. At stated periods in spring and autumu 

 the males appear; the females of the generation in which occur the 

 males have a tendency to produce eggs of a different sort charged with 

 a different mission. At the same time the upper portion of the shell 

 (that surrounding the brood cavity) becomes finely reticulated and 

 pigment is deposited between its layers. This ephippium, as it is 

 called, in allusion to its saddle-like form, is the case in which the 

 winter egg is to pass the period of cold or drought which is to follow. 

 The method of the formation of the ephippium is obscure and, in spite 

 of the investigations of Lubbock and Smitt, considerable remains to 

 be learned with reference to this interesting modification of the shell. 

 Some rather careful study has been devoted to this subject by the 

 writer, but it was unfortunately interrupted before completion. The 



