TABELLARIA. 405 



view frustules dilated in the centre by the canal, 2vith 

 rounded extremities. 



Diatoma jlocculosum Ag., Syst. p. 4. ; Kiltz. in Linna^a, 

 1833, p. 584. 1. 17. f.67. ; Hook. Br. Fl. vol. vii. p. 406.; 

 Harv. Br. Alg. p. 202. Conf, flocculosa Dillw., Conf. 

 t. 28. ; Eng. Bot. t. 1761. Bacillaria tabellaris Ehr., 

 Infus. p. 199. pi. XV. f. 7. Diatoma Jloccidosiati ^alfs, 

 in Annals, vol. ix. pi. ix. fig. 3. Bacillaria Jlocculosa 

 Ehr., Infus. t. xv. f. 9. B. seriata Ehr., Infus. t. xv. 

 fig. 8. 



Hah, Common in pools, &c. 



This is a very variable species, and it is not to be wondered 

 at that a difference of opinion should have existed as to 

 whether the conditions of it represented in " Eng. Bot.'*' and 

 in Dillwyn's " ConfervaB " were not in reality distinct species. 

 That they are not so, however, is certain, as well as that both 

 are different stages of the growth of the same production. 

 In the young state, the cells are as long and even longer than 

 broad^ each side of the frustule being marked with often as 

 many as seven well-marked striae ; and the central canal is 

 large and circular. From this state intermediate specimens 

 are often met with, some having the fru.-tules once and a half, 

 others twice, and others thrice, and even many times as broad 

 as long. Corresponding with this gradual lateral enlargement 

 of the frustules, we find the number of the striae to diminish 

 gradually, until at last but one or two exist on each frustule, 

 and at the same time, a gradual diminution in the size of the 

 central canal occurs, until at length this is nearly obliterated. 

 Thus the ultimate stage of the species bears some resemblance 

 to Tahellaria fenestrata, and is the Bacillaria seriata of 

 Ehrenberg. For these changes see the figures. 



2. Tabellaria fenestrata. 



Plate XCYI. Fig. 10. 



Char. Frustules fow?' or five times longer than broad ; in end 

 view subinfiated in the centre with incrassated ends. 



D D 3 



