418 Dr. J. E. Gray on the Arrangement of the Families 



generated by the conjunction of two distinct individuals." (See 

 Berkeley, p. 120. f. 31, 32.) 



" All writers agree in their increase by the partition of the 

 mother-cell, accompanied by the growth of two new half-cells : in 

 many cases this division goes no further, but each half with its 

 new lobes grows into a perfect whole, and again divides. But 

 this is by no means the case in all ; for the two original halves 

 do not always separate, but remain united with their progeny 

 for many generations, thus forming a filiform body, in which the 

 two primary halves are at each extremity and the youngest in 

 the middle of the thread, — a mode of increase which we shall 

 meet with again in the following tribe." (Berkeley, p. 17. 

 f. 7, p. 121. f.30«,6.) 



There is no difficulty in pointing out the passage between the 

 Desmidiese and the Zygnemidse ; for the genus Spirotania has 

 all the appearance of being a separate joint of a Zygnema. 



Fam. 3. Diatomacese. 



Endochrome yellow-brown. Cells covered with a silicious 

 coat, often beautifully sculptured. Propagation and division of 

 cells as in Desmidiacese. 



"As in Desmidiacese, there are solitary species, and others 

 grouped into lines and membranes ; and in a few which have 

 been observed to produce new plants by means of spores, the 

 new production does not exhibit at first the normal character of 

 the species." (Berkeley, p. 124.) 



" The Diatomacese were long believed to be animals, and this 

 view has more especially been maintained by Ehrenberg ; but 

 the discovery of the coupling of fronds by Mr. Thwaites, and 

 the confirmation of this fact by myself, Mr. Broome, and others, 

 leave no doubt that they belong to the same order of beings as 

 Desmidiacese and Conjugatre." (Berkeley, p. 126.) 



The genera have been divided into three groups : first, those 

 with smooth and transversely striated frustules ; the second, with 

 vittate, and the third with areolar frustules (p. 129). 



The British species have been well described and beautifully 

 figured in Dr. Smith's work, and the whole subject has been 

 revised in Pritchard's ' Diatornaceee •' but it appears to me that 

 the arrangement of Smith is more in conformity with nature 

 than that now proposed, which appears to be intended solely to 

 enable the microscopist to name the species in the easiest man- 

 ner from the isolated or prepared specimens that come into his 

 hands. 



Subclass II. POLYPHYTES. 



The zoospores aggregated into a definite form, and often 

 covered with a membranous or gelatinous coat; each zoospore 



