246 Proceedings of the Royal Irinh Academy. 



acceptance as a necessary inference from the fact of tlie continnons 

 existence of numerous species, despite of tiie law wliicli regulates 

 their multiplication by the process of self-clivision. As in each suc- 

 cessire act of fission the newly-formed valves are smaller than those 

 •within which they have been secreted, the species would soon become 

 extinct, were there not a provision made for its perpetuation in 

 the process of sporangial reproduction. All the cii'cumstances consi- 

 dered, I am led to regard the Diatomaceae as a group of organism on 

 which the Creator has impressed certain distinctive characteristics fi'om 

 which, through countless, successive ages, they have shown no ten- 

 dency to depart. 



LIST OE SPECIES. 



A. Frustules symmetrical. 1. Valves circular. 



FAZiULTl. jIELOSIRE^, Ktitz. 



Frustules simple, or adhering iu filaments. Cii'cular on side view. 



This family, since the adoption of it by Kiitzing, has undergone 

 considerable modification in respect to the genera included within 

 it. If we omit the ill-defined genus Pyxidicula, the forms he em- 

 braced within it, with the exception of Cyclotella, belonged to those 

 genera distinguished by the filamentous character of their growth. 

 Kiitzing recognised the analogy between these genera and those of 

 which Coscinodiscus may be regarded as the type, but placed them 

 widely apart, principally on the ground of the areolate striation of the 

 latter. This character, however, is by no means universal, and even 

 if it were, could scarcely justify so great a dislocation. Grrunow, 

 therefore, who is followed by Heiberg, includes among the Alelosii-e^e 

 all the symmetrical forms ciiTular on the side view, ii-respectively of 

 their peculiarities of striation ; thus establishing a very distinct and 

 well-defined gTOup which I adopt — my only difficulty in doing so aris- 

 ing from the fact that in the genus Cyclotella, some of the included 

 species are waved on the front view, and for this reason can scarcely 

 be considered as symmetrical in all aspects, in the sense of Grunow 

 ■and Heiberg. 



Genus I. ]\lEL0srKA, Agardh. 

 Frustules filamentous. Convex at the ends, filaments fi'ee. 



Melosira borrerii, (Greville.) jlarine or brackish water. 



Yalves sub-hemispherical; gii-dlebands mai-ked with conspicuous 

 •circles of cellules; filaments varying in breadth; colour' of the desic- 

 cated filaments, a rich brown. (PL 26, fig. 1.) 



