56 CONNECTICUT GEOL. AND NAT. HIST. SURVEY. [Bull. 



FAMILY I. DESMIDIACE^. 



Cells of various forms, but symmetrical, single, or, in a few 

 genera, loosely united into threads. Often a constriction in 

 the middle of the cell divides it into halves, called semi-cells ; 

 the cell contents nearly always divided. The cell wall is often 

 provided with granules, spines, or other protuberances, which 

 are usually arranged in a definite pattern. These projections 

 serve as a protection from aquatic animals, and as an 

 anchor in times of flood. Conjugation of two cells results in 

 zygospores. Asexual multiplication by transverse division or 

 separation of semi-cells. The Desmidiacece are surrounded by 

 a mucus exuded through pores in the cell wall, and often may 

 be found embedded in a mass of jelly. 



They are not free-swimming, but are able under certain 

 conditions of light and gravitation to glide along a hard sur- 

 face. 



Desmids are extremely abundant the world over; there are 

 several thousand known species, living in all degrees of tem- 

 perature, and found most abundantly in soft water. 



The following key, modified from West, includes all the 

 known genera of Desmids, four of which (Ichthyocercus, 

 Triploceras, Phymatodo is, and Streptonema) are exclusively 

 tropical, and one (Ancylonenta) is exclusively arctic. We 

 have identified certainly as yet only a part of the Connecticut 

 species, but have indicated by an asterisk the genera, and have 

 figured the species thus far observed. 



Key to Sub-families. 



Cell wall not evidently divided into two parts, and 

 without pores saccoderm^e 



Cell wall showing two segments, and with a differ- 

 entiated outer porous layer placoderm^: 



SUB-FAMILY I. SACC0DERM.E. 



Cell wall unsegmented and without pores. Point of division 

 of cells indefinite, and unknown previous to the actual division. 

 The young half of the cell is developed obliquely, and its walls 

 are absolutelv continuous with the walls of the older half. 



