34 MEMOIKS OF THE NATIONAL ACADEMY OF SCIENCES. 



A. communis, only narrower, and with longer setae, especially the terminal ones. The first antennae 

 are fourteen-jointed, while the olfactory rods and auditory bristles are essentially as in A. communis. 

 Mr. Forbes writes me that " there is some room for supposing that my A. intermedins is a 

 dwarf form of A. communis ; but I think no one could suppose that A. brevicauda was other than 

 a distinct species. It occurs in the hill country of southern Illinois, and probably also farther 

 south." 



Asellus brevicauda Forbes, I. c, p. 8. 



In this form, which lives in clear rocky rills in Illinois, which is described in detail by Mr. 

 Forbes, the first antennae are fifteen -jointed. The auditory bristles are well developed, and there 

 are five on the end of the second joint, where three are usually observed. The last olfactory rod 

 is twice as long as the terminal joint of the antenna. The first pair of legs are shorter than in A. 

 communis, while the head is much smaller, and of an entirely different shape. The caudal uropoda 

 are dwarfed, very short, but the setae much as in A. communis. 



Remarks. — It follows from the foregoing statements that the geographical range of Ccecidotcea 

 stygia is as great or greater than that of any other cave animal, unless Pseudotremia cavernarum 

 be excepted. It also appears that upon the whole it does not vary much, being iuvariably white 

 in color, blind, usually with no traces of eyes, and of a narrow, elongated shape. The parts which 

 vary most are the organs of sense — i. e., the first and second antennae, especially the first pair. 

 Its parent form is evidently one of the species described from Illinois, and as Asellus communis 

 is widely distributed over the Mississippi valley aud the Atlantic States, we are justified in 

 regarding this as the parent form. It seems to be more abundant than any other species of the 

 genus. This is evidently due to its immunity in its subterranean retreats from the attacks of 

 the host of enemies — insect, crustacean, and fish— which prey upon the eyed out-of-door forms. 

 Although blind, its loss of eye-sight is made up to i* by its greater development of sense- 

 appendages (antennae), though after all the loss of sight is perhaps of little moment, since it is not 

 exposed to the attacks of stronger animals. It breeds from April to May in Mammoth Cave, and 

 probably all summer, since it was found by Mr. Hubbard with eggs in Cave City Cave, July 29, 

 1881, and in wells with eggs in Illinois by Mr. Forbes. The number of eggs produced and kept 

 within the incubatory pouch appears to be no greater than in the out-of-door forms. 



We think we have shown that on taxonomic grounds the genus Caecidotaea is as well founded 

 as many other genera which are accepted by carcinologists, It presents, at any rate, certain 

 constant differences from the blind species of European wells and caves, as well as the dark 

 abysses of Lake Geneva, and though exposed to the same general surroundings, has developed 

 in different directions. It affords an interesting example of the origin of generic characters by 

 changes in an environment the nature of which we can easily estimate. 



Ckangonyx vitreus Smith. PL V. figs. 1 to 4. 



Stygobromus vitreus Cope, Amer. Naturalist, vi, 422, July, 1872. 



Not Crangonyx vitreus Packard, Fifth Aim. Rep. Peab. Acad. Sci., Salem, 95, 1873. 



The following description and accompanying sketches on Plate Y have been kindly prepared 

 by Prof. S. I. Smith, of Yale University : 



All the Amphipods which I have seen from the Mammoth Cave belong to a single species, 

 undoubtedly the same as the one badly described from the same locality by Professor Cope. In 

 all I have examined five specimens, collected by Professor Packard in Shaler's Brook, the Laby- 

 rinth, and Willie's Spring. 



The largest specimen, the one figured, is from Shaler's Brook,. and was found under a stone. 

 This specimen is a female, 5.2 mm in length (from the front of the head to the tip of the telson). 

 The secondary flagellum of the antennulae is minute, scarcely larger than the first segment of the 

 flagellum, very slender, and composed of two segments, of which the terminal one is very minute, 

 and about one-third as long as the first segment. The caudal stylets are all short and stout, the 

 first and second pairs, with the outer rami, a little shorter than the inner, and both armed with 

 spines which increase in length distally and at the tips are very long and slender; the third, or 

 posterior pair, are almost rudimentary, being much shorter than the telson. The basal portion 



