_ 
416 BIOLOGICAL STUDY OF TAP WATER. 
long as broad, divided in the centre by a double line; extremi- 
ties of the cells dentate ; breadth, 0:0075 mm. —0-009 mm. Fig. 1. 
The two species, Rhizosolenia Eriensis and R. gracilis, are also 
present, the former always and the latter quite frequently. As A. 
gracilis has only lately been described by Prof. Smith, by whom it 
was first discovered in filterings from the Niagara River water supply 
at Buffalo, its characters are appended :-—“ Frustules small, slender, 
round or but slightly compressed ; annuli, obsolete ; body, smooth ; 
fifteen to twenty times as long as broad ; imperfectly siliceous ; calyptra, 
conical ; bristle fully as long as the body, or longer; often slightly 
curved, and, with the calyptra, rigidly siliceous; length, 004” 
—008".” It can be readily distinguished from 2. Hriensis by its 
curved bristle, and by the absence of the markings which are so 
characteristic of the latter species. 
It might be observed here in passing that the above are the only 
two fresh water species of Rhizosolenia as yet known, all the others 
being marine. The presence of these two species, together with 
others of genera, such as Stephanodiscus and Actynocyclus, mostly 
marine, would seem to point to the fact of the connection at one 
period of the great lakes with the ocean, and the survival of a few 
marine or brackish forms, which have been able to accommodate 
themselves to the altered conditions of their habitat. 
DESMIDIACEAE. 
Desmids as far as at present known are all inhabitants of fresh 
water, and, as stated by Wood in his “ Fresh Water Algae,” prefer 
“that which is pure and limpid.” They have been found in stagnant 
water, but never in that actually putrid. Next to the Diatoms they 
are the commonest vegetable forms to be found in the filterings from 
our water supply, and they seem to be most plentiful in the latter 
part of winter and during spring. The commonest representatives 
of this family are several species of Closterium, some of which I have 
not been able to determine, 
In every gathering are to be found considerable numbers of a 
form which is figured by C. M. Vorce in a paper on the “ Microscopic 
Forms observed in the water of Lake Erie,” and called by him Clos. 
Venus, but which is much smaller than the form described by Wood 
under this name, the diameter as a general rule being not more, and 
often less, than 0-0031 mm. ( = 0°00015”"). In shape they vary- 
