ENTOMOSTRACA OF MINNESOTA. 179 
*Simocephalus serrulatus Koch. 
Daphnia serrulata—Koch, Lievin, Fischer, Lilljeborg. 
Head narrow, extending anteriorly into a sharp spiny angle in front 
of the eye. Dorsal line of the shell abruptly angled or curved pos- 
teriorly, projecting to form a broad obtuse spine behind; this spine is 
serrate with sharp teeth and lies somewhat above the middle of the 
height of the animal, so that the free posterior margins of the shell 
fall much short of reaching the greatest height of the shell. Post- 
abmomen of the usual form, with the claws armed with two series of 
spines or bristles, the outer being much the larger; anal teeth curved 
or angled, dentate; pigment fleck triangular or rhomboidal. Length 
2.0 to 2.5 mm. 
I am not sure that the three following species are more than varie- 
ties; the first in particular is very close to the Huropean type. 
[* Simocephalus americanus Birge. ] 
PLATE XLV, Fie. 9: 
My own observations of this form made throughout the Mississippi 
Valley are not in complete accord with the description of Birge, but 
it seems improbable that there is any mistake in the identification. 
The very generally distributed form on which this species rests is 
subject to marked variations within certain limits. This species dif- 
fers from S. serrulatus in the following points. The head, although 
prominent and spiny near the eye, is not angled between this promi- 
nence and the beak; in fact, it is either straight or simply curved. 
The pigment fleck is usually rhomboidal and only oceasionally oval, 
triangular or irregular. In other respects the agreement is rather 
close; the terminal claws have two series of spines, one of which is 
larger (not, as said by Birge, equal); the outer series is not so much 
larger as in S. rostratus, but not nearly as inconspicuous as in S. vetulus. 
The terminal claws are rather evenly curved. This species is frequently 
colored with pink or brown markings. In old females the back is 
squarely angled above, forming a pocket for the eggs. The size falls 
short of that of the last speeies. I have found this species from the 
Gulf of Mexico to Minnesota. Professor Birge now recognizes this as 
S. serrulatus. 
*Simocephalus rostratus Herrick. 
Herrick 784. 
This form is of the size and color of S. americanus, and approaches 
nearest to Schoedler’s S. exspinosus in general characters. The back is 
arched above but not abruptly angled; the spine is as in S. americanus 
but not solow. The free posterior shell margins are somewhat shorter 
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