ENTOMOSTRACA OF MINNESOTA. 181 
ciently llke Simocephalus. 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 Scapholeberis 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. Simocephalus 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 autumn 
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 
