ANIMAL NATURE OV DIAT0MEa3. 463 



furnished with terminal perforations. And here let lis 

 not forget how these canals evidently project into the 

 cavity of the Melosirese, forming more distinct vittse than 

 others ever do ; on which account, if we ought really to 

 found on this character the distinction of orders, the 

 family of Melosirese or the genus Melosira at the least, 

 ought undoubtedly to be referred to the order Vittatse. 



46. RfliPiDOPHORA. — Bacilli a latere primario cuneati, 

 altera latere ohovato-lanceolati, stipitati. 



We encounter the same difficulty in distinguishing 

 RJiipidophora from Podosphenia that is experienced 

 when practically applying the generic distinction estab- 

 lished between Sphenella and Gomphonema, and in all 

 other similar cases {Cymhella and Cocconema, &c.) ; these 

 differ only in the stipes, which is very variable in length, 

 and not always entirely wanting in the first of these 

 two genera. 



The large size of some among the fourteen species 

 enumerated by Kiitzing permits us to observe clearly 

 the conformation of the shield. Let lis suppose a cylin- 

 drical articulation oiMelosira, and so compress it unequally 

 on one of its sides, and in the direction of both pairs of 

 opposite surfaces, that the resulting form shall be cuneate, 

 and the two incomplete diaphragms formed by the 

 internal prominence of the longitudinal canals shall 

 extend, like these, and lose themselves towards the pointed 

 extremity which forms the base. Such is the structure 

 of Podosphenia and Bhipidophora. Viewed on one side, 

 that is on the lateral surfaces, they present an obvate 

 arch {fornice) marked on the periphery of the surfaces 

 themselves. And the margin of this arch is thickened 

 by the presence of the canal, which, seen in front, presents 

 in the curve its brightness, with an appearance of 

 perforation. 



It would be important to ascertain how the dedupli- 

 cation takes place. Whenever one of the frustules, 

 (Bacilli) separates into two, one of the vittse previously 



