1883.] Heterogenetic Development in Diaptomus. 501 



Plate vii). 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. i 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 xi Figs. 

 15-16 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 sirhply 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. 



The 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. Icevis 

 of Birge in which the development is tolerably well completed. 



By filling up the gaps till all the stages in each case are known, 

 we have advanced one step toward an accurate determination of 

 species, and require then to learn what variations in the process 

 usually obtaining may be occasioned by alterations in the envi- 

 ronment. But in the mean time we are discovering the laws 

 which govern development and the historical affinities of the dif- 

 ferent genera and species. 



Daphnia pulex. 



This common species is subject to variations which are per- 

 plexing, but there seems to be no reason for the separation of the 

 var. denticidata, as done by Birge. The differences relied upon are 

 the fine teeth down the claw and the abrupt curvature of the 

 lower margin of the head. Fig. 4 of Plate i in P. E. l^ueller's 

 work figures this peculiarity of the claws. Quite typical D. pulex 

 from Tuscaloosa, however, do not have the fine teeth and only 

 12-15 teeth on the abdomen. 



