120 DUBLIN UNIVERSITY 



form and size of the species. The straight outline of this form, as, 

 indeed, I need hardly point out, at once excludes it from Closterium, 

 while it is, of course, equally decisively distinguished from Docidium 

 by its tapering form, rounded (not truncate) ends, and by its want of a 

 central constriction and its non-inflated segments; — from Spirotacnia 

 (in the recent state at least), its scattered, non-spiral endochrome at 

 once removes it ; — the want of a constriction and terminal notch excludes 

 it from Tetmemorus ; — while, from all the foregoing circumstances, it 

 will be seen that it is really a Penium. From the striate, or granulate 

 species of Penium, it may at once be known by its smooth frond, from 

 which, indeed, its attenuated ends would sufficiently distinguish it. Of 

 the species with smooth fronds, it appears to have greatest affinity to 

 Penium closterioides (Ralfs), by reason of its terminal cavities with mov- 

 ing granules, and its fusiform outline. But, in the form in question, its 

 very much smaller dimensions, combined with the entire absence of, or 

 sometimes faintly apparent, longitudinal fillets, as well as the presence 

 of only two conspicuous dense corpuscles (not a longitudinal series), its 

 more cuneate segments, and its more narrow and slender ends, to which 

 the terminal cavities are closer, readily distinguish it from that species. 

 From the other smooth species, except Penium interruptum {Brib.), the 

 presence of the terminal cavities, containing active granules, at once 

 removes it, while it never could be mistaken for that species (with which 

 it is, indeed, unnecessary to compare it), on account of its far more 

 minute size, as well as the absence of the two additional transverse 

 bands, and its fusiform (not cylindrical) outline. Neither can it be 

 mistaken for Cosmarium curtum (Ralfs) = Penium curtum (Breb.), from 

 which it is at once separated by its smaller size, less inflated appearance, 

 the want of any central constriction and of so conspicuous longitudinal 

 fillets, as well as by the possession of the terminal cavities, containing 

 moving granules. 



There is only one other form with which I need particularly contrast 

 it, and that is SpiroUenia obscwra {Ralfs). At first sight it might appear 

 unlikely to be mistaken for that species, nor is there any resemblance 

 when fresh specimens of both are examined. Then the spiral arrange- 

 ment of the endochrome in Spirotaenia alone is an abundantly sufficient 

 mark of distinction ; but when Spirotaenia obscura is kept for some time 

 in the house, this spiral disposition of the endochrome is lost, and it be- 

 comes uniformly green. Moreover, there not unfrequently occurs in 



