North American Species of Diaptomus. 169 



Diaptomus ashlandi is very similar to D. sicilis Forbes and 

 hardly to be distinguished from it but for a slight difference 

 in the last segment of the left fifth foot of the male and the 

 position of the marginal spine of the right fifth foot. In the 

 Laboratory collections from Yellowstone Park and the Flat- 

 head region of Montana forms occur which seem to be inter- 

 mediate between the two, and it was exceedingly difficult to 

 decide to which, if either, of the two species they belonged. 

 D. ashlandi seems to me, however, to be a good species, since 

 the form, as described by Marsh ('93 and '95) and as found 

 by myself in other collections, exhibits constant, though some- 

 what minute, differences from sicilis hard to describe, but at 

 once evident from the figures. 



A very peculiar modification of the inner ramus of the fifth 

 foot of the female was noted in a specimen taken from an 

 alkaline pond in Yellowstone Park. In this individual one of 

 the feet (PI. XXXII., Fig. 1) was normal in every respect, 

 while the inner ramus of the other was armed on its outer 

 margin, at the end of the proximal third, with a sharp, smooth 

 spine about half as long as the ramus itself. 



D. sicilis var. imperfect us Forbes ('90) is here made a syn- 

 onym of D. ashlandi, because unpublished Laboratory draw- 

 ings of that variety clearly show it to be such. Marsh's de- 

 scription must stand, since the description of imperfectus was 

 not complete enough to identify the form. This species was 

 also noticed by Dr. Forbes in the collections reported on in 

 '93, but was erroneously regarded by him as an immature 

 form of D. sicilis. 



Diaptomus reighardi Marsh. (PI. XXVIII. , Fig. 1.) 



Diaptomus reighardi, Marsh, '95, p. 9, PI. I., Fig. 1-4. 



A medium-sized species ; body about the same width 

 throughout. Sutures between the first and second, and between 

 the last two, thoracic segments distinct ; last thoracic seg- 

 ment not produced, armed on each side with a very minute 

 spine. First abdominal segment almost as long as the rest 

 of the abdomen, dilated laterally, armed on each side with a 



