TABKLLARIA. 405 



view frustules dilated in the centre hy the canal, with 

 rounded extremities. 

 Diatoma jioccnlosum Ag., Syst. p. 4. ; Kiitz. in Linnaja, 

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

 Harv. Br. Alg. p. 202. Conf. focculosa Dilhv., Conf. 

 t. 28. ; Eng. Bot. t. 1761. Dacillaria tahcllaris Elir., 

 Infus. p. 199. jil. XV. f. 7. Diatoma focculosn)n liar's, 

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

 Ehr., Infus. t. xv. f. 9. B. seriuta 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 Dillwyu's " Confervas " were not in reality distinct species. 

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

 are difl'erent stages of the growth of the same production. 

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

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

 many as seven well-marked stria3 ; and the central canal is 

 large and circular. From this state intermediate specimens 

 are often met Avith, some having the frustules 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 striaj to dimini.<h 

 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 fcncstrata, and is the Bacillaria scriata of 

 Ehrcnbcrg. For these changes see the figures. 



2. Tabellakia fenestkata. 



Plate XCVI. Fig. 10. 



Char. Frustules four or Jive times longer than broad; in end 



view subinjiated in the centre icith iiicrassated ends. 



D n 3 



