1 6 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, Rhizosolmia Eriensis and R. gracilis, are also 

 present, the former always and the latter quite frequently. As R. 

 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 calypti^a, rigidly siliceous ; length, "004" 

 — -008"." It can be readily distinguished from R. Eriensis 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. 



Desmicls 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 Glos. 

 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 



