STATE GEOLOGIST. 49 



species. This species becomes over 1-10 inch. long. In such old 

 individuals the spine is nearly midway of the hight. 



One could wish a trifle closer link to Scapholeberis than that furn- 

 ished 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 too closely related. 



Note.— On p. 47 read S. Americanus, Birge, not S. Congener. 



V. Genus Daphnia. 



Long considered the type of the family, this genus is most fre- 

 quently 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, Simoceph- 

 alus is the link connecting it with the typical forms of the fam- 

 ily. As might be expected, this genus presents more puzzling 

 problems than any of the others. It contains more peculiarities 

 of structure and diversities of habit and development than any 

 other of the genera. Here the sexual differences are most inter- 

 esting. The young are hatched with a pendant appendage at- 

 tached to the upper posterior angle of the shell, which soon be- 

 comes 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 pro- 

 cesses, of which the first is much the larger and curves forward. 

 At stated periods in spring and autumn the males appear; the fe- 

 males of the generation in which occur the males have a tendency 

 to produce eggs of a different sort charged with a different mis- 

 sion. At the same time the upper portion of the shell (that sur- 

 rounding 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 re- 

 mains to be learned with reference to this interesting modification 

 of the shell. Some rather careful study has been devoted to this 

 4 



