222 TENTH ANNUAL REPORT ^^ i 



The above bibliography is complete only up to a comparatively re- 

 cent date; many notices may be found in recent literature. 



The species which, after careful study, has been considered identical 

 with the European D, castor is that described previously as D. longi- 

 cornis with the remark that it might prove too near D. castor. D, san- 

 guineus^ Forbes, seems to be the same thing nearly. If the amount of \ 

 variabilit}' admitted by Brady to prevail is allowable I see no reason 

 for separating this species. It is very variable as to size and colora- 

 tion, and even in the configuration of some of the parts, as antennae, 

 etc., a certain amount of latitude is to be given; (See plates of Claus, 

 Zur Anatomic und Entwicklungs-geschichte der Copepoden Arch. f. 

 Naturg. XXIV Jahrg., B [.) tVo-tVit; cm. in length. The following 

 points are variable : — length of caudal stylets, structure and thickness of 

 male geniculating antenna, size of claw of fifth male foot and spinous 

 armature of feet. How far such variations may extend and how much 

 they are dependent on peculiarities of habitat, etc., farther study must 

 demonstiate. Some interesting facts meanwhile are suggestive. A 

 second and gigantic form which may be known as 



Diaptmnus giganteus. 



(Plate II, figs. 3-11-15.) 



with the reservation that it is doubtfully of more than varietal value, 

 was found under such circumstances as to suggest that it might be 

 only a curiously magnified condition of D. castor. It is known to 

 occur only in a small marshy pool of about two square rods extent and 

 which annually dries up nearly completely. A few yards away is a 

 second pool of a ^mewhat greater size and which less frequently dries 

 up in summer These two pools within the memory of the writer 

 were united, but in the gradual dessication which has been observed in 

 all Minnesota, they have been isolated. The former pool in June was 

 found to contain mature males and females of the D. giganteus only, 

 few other copepods being present, while the other pool contained all 

 stages of the common D, castor. There are no neighboring waters, the 

 nearest being half a mile, and that (L. of Isles) has only D. castor. 



The smaller pool soon completely dried up so that this form was, for 

 the time, exterminated. The conclusion seems almost resistless that 

 the stagnation incident to evaporation produced circumstances favor- 

 a])le to the development of this enlarged form. At any rate it is an 

 interesting fact in local distribution 



This variety is fif cm. long or more, and i^ much the largest fresh 



