I). .1. SCOUHFIELD UX ENTOMOSTRACA. 1 67 



indefinite character. One of the most noticeable of these is 

 the curious non-appearance of some species of Cladocera in 

 1891, e.g., Sida crystalUna, Alona costafa, and Alonella nana. 

 This is the more strange because each of these had been pre- 

 sent in 1889 as well as in 1890, and they are all forms that 

 occur pretty plentifully when they do put in an appearance. 

 The first named, too, is one of the largest of the Cladocera, and 

 it seems quite impossible to believe that it, at any rate, could 

 have been overlooked. The most plausible hypothesis to ac- 

 count for their absence seems to be that the unusually severe 

 winter of 1890-91 proved fatal to them in some way or other. 

 In the case of Sida crystalUna this view receives some apparent 

 support from the fact that the resting-eggs of this species, upon 

 which it entirely relies for continuance from year to year, are 

 dropped to the bottom of the pond, covered only by their own 

 proper membrane, and not enclosed in a protecting " ephip- 

 piiim " as among the Daphnidae. It seems likely enough, 

 therefore, that they are the more susceptible to extremes of 

 temperature. The particular spots, too, where this species 

 occurs in Wanstead Park are comparatively shallow, and even 

 the bottom mud may have been frozen solid by the prolonged 

 frost. In this case the pressure exerted upon all resting-eggs 

 would be enormous, and they would, no doubt, suffer \Qvy 

 greatly if not protected by such extrinsic envelopes as ephippia, 

 which would at least serve to equalize this pressure, and 

 probably to counteract it altogether if containing air-spaces. 

 But how this explanation can be reconciled with the fact of 

 non-appearance one year and reappearance at the same spot in 

 the next is more than I can undertake to say. One thing at 

 least the hard winter of 1890-91 was responsible for, if not for 

 the non-appearance of the speciei-j above-mentioned, namely, 

 the almost entire absence of Cladocera on the 31st January, 

 1891, when only a single individual of Simocephalus vetulus 

 could be found. 



The periodical occurrence of Cladoceran males and " ephip- 

 pial " females is another point of considerable interest, and one 

 that must certainly be carefully attended to in future records. 

 Unluckily, in this case, no attempt was made until August, 

 1891, to separately note these sexually mature individuals, and 

 even since tliat date the records must be very incomplete in 



