1883. ] Heterogenetic Development in Diaptomus. 501 
Plate vil). Fig. 3 is the embryo extracted from the brood cavity, 
showing that the eye is near the extremity of the elongated head 
even before the two eye spots have united (Fig. 10). Fig. 2 
shows the early stage of post-embryonal growth. Fig. 1 is the 
adult in the first stage in which all the peculiarities of the species 
are pronounced. Fig. 4 is the post-imago (see also Plate x1 Figs. 
{5-16 of Notes on Minnesota Crust., and Fig. 4 in the text). 
DAPHNIA DUBIA, sp. nov. ? 
This species is very nearly related to the preceding, differing, 
however, in having the eye small and situated nearly in the cen- 
ter of the head, while the previous one has an eye of usual size 
and near the straight lower margin. The head is much more 
acute than in the preceding but not carinated (Fig. 9). Both 
these species have occasionally a horn in young stages (Fig. 14). 
The claws are smooth or simply pectinate. The spine in the lat- 
ter species is more elevated. It is remarkable that the same 
Species has both forms of abdominal appendages represented at 
Figs. 13 and 8, 
ton ane post-imago of dubia has not been seen. The nearest ap- 
Proach to it had a considerable spine, but the head had already 
begun to assume the shorter form with a curved lower margin. ` 
The only allied species yet described from America, is D. Jevis 
of Birge in which the development is tolerably well completed. 
By filling up the gaps till all the stages in each case are known, 
a a advanced one step toward an accurate determination of 
Species, and require then to learn what variations in the process 
Mealy. obtaining may be occasioned by alterations in the envi- 
` arg But in. the mean time we are discovering the laws 
d ich govern development and the historical affinities of the dif- 
a Rrent genera and species, 
Darnsra PULEX, 
ah common species is subject to variations which are per- 
‘ iy but there seems to be no reason for the separation of the 
| ~~ K lata, as done by Birge. The differences relied upon are 
toes: ett down the claw and the abrupt curvature of the 
“Work margin of the head. Fig. 4 of Plate 1 in P. E. Mueller’s 
a from Tuscal this peculiarity of the claws. Quite typical D. pulex 
ts ia: however, do not have the fine teeth and only 
fst on the abdomen. 
