BRITISH DIATOMACEJS. 53 



present in all the Melosirece, but eminently conspicuous in Poclosira 

 and Melosira, retaining the frustules in these genera, after self-divi- 

 sion, in a geminate union until the self-dividing process is renewed. 



The brevity of the filaments, which rarely include more than from 

 two to six frustules, the absence of silex at the apex of the valves, 

 and its parasitic growth, are characteristics which place the genus 

 Podosira apart from its allies. This deficiency of silex is probably to 

 allow a free secretion of the mucus which unites the frustules, and 

 provides a pedicel for their attachment to the plant on which they 

 grow, as it does not occur in the non-attached valve of the first- 

 formed frustule. In the living state, the absence of the silex is not 

 perceived ; but when the frustules have been macerated in acid, these 

 portions of the valves appear as perforations, owing to the disappear- 

 ance of the cell-membrane. A similar secretion of mucus takes place 

 in Melosira nummuloides /3. (Plate XLIX. 329 /3.) ; but in this case no 

 deficiency of silex is apparent, and no perforations are seen in any 

 condition of the valves : the cause of this diversity of structure, 

 under circumstances so similar, I cannot explain. 



1. Podosira Montagnei, Kiltz. Filaments usually of two frustules, 

 which are at first compressed at the equator; cingulum firmly 

 siliceous, with annulate striae. Breadth of filament '0018" to 

 •0028". v.v. 



Kiitz. Bacill. xxix. 85. Melosira globifera, Harv. Man. p. 196? Ralfs, 

 Ann. vol. 12. ix. 3. ad specim. authen. in herb. Jenn. Rosaria globi- 

 fera, Carm. apud Grev. B. F. p. 3/2 ? 



Marine. Hastings, Mr. Jenner. Torbay, July 1846 ; Poole Bay, Nov. 

 1849 and Sept. 1851, W. Sm. (Near Jaffa, Levant; communicated by Mr. 

 Johnson.) 



Plate XLIX. 326. 



2. Podosira hormoides, Kiitz. Filaments of from two to six 

 frustules, which are at first compressed at the poles; cingulum 

 obscurely punctate. Breadth of filament *0007" to '0009". v.v. 



Kiitz. Bacill. xxviii. 5. & xxix. 84. Prit. Anim. xiii. 45. 



Marine. Pevensey Bay, Sussex, Aug. 1850; Poole Bay, Sept. 1852; 

 Stomach of Pecten, Coast of Sussex, Mar. 1853, &c, W. Sm. Frith of Clyde, 

 Feb. 1854, Mr. Hennedy. 



Plate XLIX. 327. 



