144 THE CANADIAN NATURALIST. [June 



ia the genus Pleurosigma, — some maintaining that they were 

 depressions, others regarding them as elevations. The Rev. Mr. 

 Reade has at last, I believe, settled this vexed question by means 

 of a very simple little piece of apparatus, which he calls a 

 " Diatom prism." With this he clearly demonstrates that the 

 dots into which the striae are resolvable in the Pleurosigma 

 and allied genera are elevations, and he aptly compares them 

 to a "field of haycocks," or a "plate of marbles." Through the 

 kindness of Prof Bovell, I have been enabled to use the prism, 

 and certainly the appearances produced strongly favour this view. 

 The large cellular markings of some of the marine genera, Isthmia 

 for example, are undoubtedly depressions, while in the beautiful 

 genus Pinnularia, the stria3 are continuous, and have been called 

 by Mr. Thwaites, "costae." 



Minute apertures exist along the line of suture, and at the 

 apices, through which the cell is nourished; these, in some genera, 

 communicate with the interior by means of channels (canaliculi) 

 hollowed out between the valve and primordial utricle. Nodules 

 of silex are present at the centre and extremities of the valves, in 

 many species. These, by Ehrenberg and others, were supposed 

 to be apertures ; but they probably only serve to give additional 

 streD2;th and firmness to the valves. 



Increase in the Diatomaceae takes place in several ways, namely 

 — by division — by conjugation — and, most likely, by the forma- 

 tion of gonidia. The first of these methods is, as Mr. Thwaites 

 observes, rather an act of generation than of reproduction. It is 

 thus described by Messrs. Grriffith and Henfrey — " The primor- 

 dial utricle, enclosing the contents, divides into two portions, which 

 separate from one another in a plane parallel with the sides of the 

 undivided frustule ; the two halves of the parent cell gradually 

 separate from one another, remaining connected by the simulta- 

 neous gradual widening of the hoop. In the space thus aiforded 

 the two segments of contents secrete each a new layer of mem- 

 brane (ultimately silicified) over the surfaces where they are in 

 contact, which layers of membrane constitute the two new half 

 frustulcs, back to back, corresponding to and conjoining with the 

 two half frustules of the parent, to form new individuals." In 

 the free species, when the self-division is completed, the hoop 

 drops off — and it is not until that occurs, that the two new half 

 frustules become perfectly silicified ; but in the filamentous species 

 the hoop is persistent, and forms the connecting band between the 



