148 BRITISH DESMIDIE.E. 



jection, wliich is wanting in Tetmemorus Brebissonii, and the puncta seen in 

 its empty frond are not arranged in longitudinal rows. 



Tetmemorus granuJatus differs from T. Icevis in its less constricted middle 

 and its larger size. Its puncta too are usually Tery distinct, and the discovery 

 of their respective sporangia leaves no doubt that these plants are distinct. 



Its sporangia have been gathered by j\Ir. Jenner in Sussex, and by myself 

 near Dolgelley. They differ considerably from those of T. Icevis ; they are 

 orbicular, and are not inclosed in a quadrate cell, but have the empty segments 

 of the conjugating fronds loosely attached by an imperceptible membrane ; the 

 margin of the sporangium is finely striated, a character which I have not 

 noticed in Tetmemorus IcBvis. 



Length of frond from y-Lg. to yi^ of an inch ; breadth -^ ; breadth at 

 constriction from yts ^^ "rrr 5 diameter of sporangium from 3^ to -3^. 



Tab. XXIV. fig. 2. a. front view ; b. side view ; c. empty frond. 



14. PENIUM, Br^b. (in lit.) 



Frond simple, elongated^ straight^ not, or but slightly, constricted ; 

 segments entire. 



The frond is cylindrical or elliptic and straight, with its opposite 

 margins similar, its constriction none or very slight, and the suture 

 at the junction of the segments is usually either indistinct or wanting. 

 The segments are quite entire. 



M. de Brebisson has constituted this genus for some plants which 

 he separated from Closterium on account of their straight fronds, 

 which have the opposite margttis similar. It differs from Tetmemorus 

 by the absence of a terminal notch, and from Cosmarium by its more 

 elongated frond and its less marked constriction at the junction of the 

 segments ; yet the cylindrical species of Cosmarium closely approach ' 

 to this genus, and Cosmarium turgidum and Cosmarium attenuatum 

 are at least as much elongated as Penium truncatum. 



Although the species in Penium are few, they present considerable 

 differences in their characters. Some are smooth, and, like Closterium, 

 have the endochrome arranged in longitudinal fillets, and at each ex- 

 tremity of it a globule containing moving granules. Other species 

 have a granulated surface, and there are two smooth and truncate 

 which some botanists have referred to the Palmelleae. 



Penium Digitus, on account of its size, is represented as magnified 

 only 200 diameters. 



