AQUATIC INVERTEBRATE FAUNA OF WYOMING AND MONTANA. 225 



In the open water there was always a very fair supply of entomostraca, both . 

 Cladoccra and Diaptomi, .but at the time of our arrival on the West Bay the phenom- 

 enal fact was the vast abundance, both in deep and shallow water, of & rotifer or 

 wheel animalcule which forms rolling spherical colonies imbedded in a gelatinous 

 medium, each colony consisting of a little cluster of these animalcules arranged in 

 such a man uer that their inner ends approach each other in the middle of the mass, 

 while their outer ends, with mouths, cilia, etc., are exposed on the surface. To the 

 naked eye these colonies of rotifers appear like minute grayish specks of floating 

 matter. This species belongs to the genus Conochilus, but differs noticeably from the 

 common C. volvox. I have thought best, consequently, to describe it as G. leptopus 

 (page 256). It was so abundant in the water that a haul of a ring net, a foot across, 

 for fifty strokes of a single pair of oars gave a measured half pint of this form alone. 



This colonial rotifer is not to be confounded with the "water bloom," which devel- 

 oped in Yellowstone Lake a little later to an extent very embarrassing to our suri'ace 

 net work. This so-called " bloom " consisted of specks of various alg;e growin g so freely 

 in the water as to give it a faint tint of dirty green, and washing ashore in quantity 

 along the leeward side of the lake, usually at this season the northern and eastern. 



Away from the shore, by far the most common crustacean was Daphnia pulex* 

 Although in ordinary situations the males of Daphnia are by no means common, 

 in our Yellowstone Lake collections, made in August and September, the males of this 

 variety were many times commoner than the females, making sometimes nearly the 

 whole of a large catch. The few examples of the other sex seen were mostly young, 

 although a female bearing the ephippium occurred occasionally. Next in abundance 

 was the smaller of the Diaptomi found in Shoshone Lake, the so-called variety of D. 

 sicilis, and with this came somewhat rarely, but still fairly abundant, D. shoshone and 

 D. lintoni. Several species of Cyclops occurred here, only a new one ( C. minnilus) 

 very frequently, however, and this in small proportion. 



Most of these crustaceans ranged in shore as well as in the deeper water of the 

 interior parts of the lake, Daphnia pulex falling away in numbers more rapidly in 

 shallow-water collections than Diaptomus sicilis. To these inshore species we may 

 add, from our surface-net collections, Polyphemus pediculus (sometimes very abundant 

 among the weeds), Cyclops gyrinus, and C. serrulatus (both rare), Chydorus sphcerivus 

 (few), Scapholeberis mucronatus (few), Cypris sp. (only occasional), Alona, and the usual 

 miscellaneous drift of shore forms, Chironomus, Allorchestes (both dentatus and inermis), 

 Oammarus, caseworms, hydrachnids, planariaus, Clepsine, etc. 



Collections made in the lake near enough to the outflow of hot springs to exhibit 

 their influence differed from those made in cold water only in their more scanty char- 

 acter; and where the water was actually warm it commonly contained nothing but the 



"The common anil oven abundant occurrence of this species in Yellowstone Lake as a form 

 apparently pelagic in its habits (widely contrasted, consequently, with its usual character) was so 

 unexpected and unusual that I hesitateil long before assigning this Daphnia to the species most abun- 

 dant in our stagnant pools. Prolonged study of it from various collections in the Park in comparison 

 with those from the waters of Illinois, has finally led me to conclude, however, that this Yellowstone 

 Lake form is not to be specifically distinguished from American examples of pulex. In order to 

 furnish material for a more critical comparison than has hitherto been made of the American and 

 European representatives of this species, I append a description, under the varietal name of pulicaria, 

 based upon Yellowstone specimens, with figures of both sexes (page 242 and plate xxxvn, fig. 1). 



F. C. B. 1891 15 



