362 DESMIDE^. 



notched nor lobed, but mostly either denticulated or dotted, 

 almost touching each other. 



Derivation. From /cocrfiapiov, a small ornament. 



The form of the frond in this genus is much more simple 

 than that of most of the other genera of Desmidece, being 

 neither notched nor lobed. The genus to which it is most 

 closely allied is Xanthidium, from which it chiefly differs in the 

 minuteness of the spines or teeth, which are placed on the 

 outer surface of the frond in most species, as well as in the 

 absence of the lateral apertures noticed in Xanthidium fur- 

 catum and X. aculeatum. Through Closterium cylindricum, 

 it exhil^its some relation to the genus Closteiium, in which 

 the form of the frond is also very simple ; but the two genera 

 are abundantly distinct. In Cosmarium the fronds are never 

 elongated nor curved as in Closterium, and they are always 

 constricted in the centre ; neither are they notched as in 

 Tetmemorus and Euastrum, nor lobed as in the latter genus, 

 nor compressed as in Micrasterias. 



The denticulations observed on the surface of the fronds 

 of most species of the genus vary according to the age of the 

 specimens, being most strongly marked on the more fully de- 

 veloped examples. 



a. Fronds denticulated. 



1. Cosmarium margaritiferum Menegh. 



Plate LXXXVI. Fig. 1. 



Char. Fronds denticidated. Segments broader than long, 

 someivhat hemispherical or reniform, comjoressed. End 

 view elliptic. 

 /S Fronds dotted, but not denticulated. 



Menegh. Synop. Desmid. in Linnrea, 1840, p. 219. Cos- 

 marium dentiferum Corda, Observ. IMicroscop. sur les 

 Animalc. de Carlsbad, p. 30. pi. vi. f 41. Euastrum 

 margaritiferum Ehr., Infus. p. 163. tab. 12. f. 7.; Pritch. 



