462 ANIMAL NATURE OF DIATOME/ii:. 



surfaces, for that condition ought to be referred, of 

 necessity, to important peculiarities of internal structure. 

 And therefore it becomes difficult to conceive how the 

 exceptions can be so numerous as to render this cha- 

 racter so insufficient for classification. 



The Stomaticse, together with the Astomaticse, of which 

 we have already spoken, constitute the great order of 

 Striated Diatomese, which is proportionally, much more 

 extensive than the two following. 



45. PoDOSPHENiA. — Bacilli a latere priniario cuneati 

 a latere seciuidario obovato-lanceolati, ajfixi. Stipite nullo 

 {vel obsoleto). 



With this genus commences the order of Diatomese 

 furnished with Vittse, or internal prominences, which 

 divide the cavity of the shield more or less incom- 

 pletely into distinct chambers ; and here, too, commences 

 the family of Licmophorese, in which is reproduced the 

 cuneate form of the Gomphonemese. This genus repre- 

 sents, in the Licmophorese, the genus Sphenella of the 

 Gomphonemese, for, like that, it is distinguished from 

 other genera of the same family by the more or less 

 complete absence of the stipes. The obovato -lanceolate 

 figure of the secondary surfaces is precisely that of the 

 Sphenella and of the Gomphonemese in general. The 

 cuneate form of the primary surfaces is, in PodospJienia, 

 always more dilated at the summit and acute at the base, 

 so that they resemble a triangle more than a trapezium. 

 Of the nine species described and figured by Kiitzing, 

 only one [P. Ehrenhergii) presents transverse strise on 

 the secondary surfaces. The essential character of 

 Gomphonemese, the median aperture in both secondary 

 surfaces, is absent. 



They present also the character of Vittse. But if we 

 carefully examine these vittse, they are merely the same 

 longitudinal lines which run along the primary surfaces 

 of almost all the preceding Diatomese, the same lines 

 which in many cases are produced by distinct canals 



