166 



Chlorophyceoe 



breadth 115 145 /x) are not uncommon in permanent boggy pools and lakes, 

 whilst M. oscitans Ralfs var. mucronata (Dixon) Wille and M. Jenneri Ralfs 

 are frequently found in the Sphagnum-bogs of mountainous areas. J/. furcata 

 Ag., M. Cnix-Melitensis (Ehreub.) Hass. (fig. 61 A) and M. pinnatifida (Kiitz.) 

 Ralfs are amongst the rarest and most elegant species of the genus. 



Fig. 61. A, Micrasteriax Crux-Melitensis (Ehrenb.) Hass., from Bowness, West- 

 moreland ( x 365). B, HI. truncuta (Corda) Breb., from Thursley Common, 

 Surrey ( x 200). C, zygospore of M. denticulata Breb., from Halgavor Moor, 

 Cornwall ( x 110). 



Genus Cosmarium Corda, 1834. [Dysphinctium Nag., 1849 ; 

 Calocylindrus (Nag.) Kirchn., 1878 ; Cosmaridium Gay, 1884 ; 

 Pleurotceniopsis (Lund.) Lagerh., 1887.] This is the largest genus 

 of Desmids, embracing several hundreds of species, which although 

 exhibiting many varied characters, all conform to a common type 

 of structure. The cells are relatively short and the length is rarely 

 more than one-and-a-half times the breadth. There is a median 

 constriction of variable depth, in some cases very slight, but in 

 others exceedingly deep. The semicells may be circular, elliptical, 

 semicircular, ovate, pyramidate, or cuneiform in general outline, 

 and the apex, which may be rounded or broadly truncate, possesses 

 no apical notch. The cell-wall may be smooth, scrobiculate, 

 granulate, verrucose, or clothed with minute spines, the orna- 

 mentation in almost all cases being arranged upon some definite 

 plan. There is frequently a protuberance of some considerable 



