272 DESCRIPTION OF [Poli/gastrica 



It may be distinguished from the preceding species by the front 

 and lateral views being similar. Length 1-1 33rd. 



Gemis PENrtJM (Breb.) — Frond simple, elongated, straight ; not, 

 or but slightly, constricted ; segments entire ; the opposite margins 

 similar ; and suture either indistinct or wanting. 



It differs from Clostermm by its straight fi'onds, with similar oppo- 

 site margins ; from Tetmemorus by the absence of a terminal notch, 

 and from Cosma/rium 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 C. turyidmn 

 and C. attenuatum are, at least, as much elongated as Penium 

 tnmcatum. 



Some species of Penium are smooth, and, like Closterium, 

 have the endochrome arranged in longitudinal fillets, and at each 

 extremity of it a globule containing mo\'ing granules. Other species 

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

 which some botanists have refeired to the Pahnellece. 



As in other instances, we follow Mr. Ralfs division of this genus. 



* Empty frond either striated or granulate, generally reddish. 



P. margaritaceum = Closterium margaritaceum (Ehr.) — Cylindiical, 

 with rotundato truncate ends; pearly granules in longitudinal 

 series. 



Var. {a.) — Fusiform, constricted at the middle ; gi-anules distinct, 

 giving a denticulate appearance to the margin. 



Var. (l.) — Linear, scarcely constricted at the middle; denticulate 

 on the margin, somewhat scattered near the suture. 



Var. {c.) punctatum. — Linear, neither constricted at the sutui'e, 

 nor denticulate at the margin ; granules appearing like puncta. This 

 last form agrees best with Ehrenberg's figure of Closterium margari- 

 taceum, but it is not denticulate at the margin. 



Length of frond, in faj 1-1 56th; in fcj 1-1 69th (P. 13, f. 14.) 



The sporangia are orbicular. (P. 13, f. 15.) 



P. oylindrus = Closterium (}j cylindrus (Ehr.) — Cylindrical ; ends 

 rounded truncate, rough, with pearly scattered granules. Length 

 l-49:nd. 



* * Frond smooth, colourless. 



\ Sporangium situated between the deciduous cells. 



