174 Chlorophycece 



There are five British species of this genus all of which are very rare. 

 C. constrictum (Arch.) Josh. (fig. 66 A) and C. pulchellum Breb. (fig. 66 B) are 

 perhaps more often observed than the others. All the species are very small, 

 the largest being C. Saxomcum De Bary; length 15 17 /x; breadth 13'5 

 14-5 /*. 



Genus Oocardium Nag., 1849. This is the most extra- 

 ordinary of all the genera of Desmids and usually occurs in large 

 colonies. The cells are small, slightly constricted and much 

 depressed, being considerably broader than their length. The 

 semicells are unequally depressed on the two sides, so that the 

 plant is symmetrical in one plane only. The vertical view is 

 broadly elliptical. There is one chloroplast in each semicell, con- 

 sisting of somewhat irregular plates radiating from a central mass 

 containing one pyrenoid. 



The colony is generally hemispherical in shape, 1 2 mm. in 

 diameter, and occurs attached to calcareous rocks, not unfrequently 

 being itself encrusted with calcium carbonate. It consists of a 

 number of more or less parallel, radiating strands of mucus of 

 considerable thickness, each strand widening out towards the 

 surface of the colony and occasionally branching. In the free end 

 of each mucous strand is lodged a single cell, disposed with its 

 longitudinal axis at right angles to the axis of the mucous strand. 

 The zygospores are unknown. 



The only known species is Oocardium stratum Nag. It is extremely rare 

 and I have only observed it in the limestone districts of West Yorkshire, 

 attached to rocks and stones in the beds of several mountain streams. Length 

 1316-5 p; breadth 18 19'5 M ; fig. 66 D F. 



Genus Sphaerozosma Corda, 1835. The cells are small and 

 attached to form long filamentous colonies, often twisted and 

 sometimes enveloped in a mucous investment. The median con- 

 striction may be deep and narrow or it may be widely open, and 

 the semicells may be elliptical, oblong, or subrectangular in form. 

 The attachment of the cells is apical and is effected by small 

 rounded tubercles or short capitate processes. The vertical view 

 is elliptical. There is one axile chloroplast in each semicell, 

 furnished with a single pyrenoid. The zygospores, which are 

 globose or oblong, are either smooth or furnished with subulate 

 spines. 



There are only five British species of the genus and none of them is 

 abundant. 8. vertebratum Ralfs (length of cells 19 /*; breadth 21 24 p; 



