BRITISH DIATOMACE^E. 17 



occurred to me with a majority of the frustules containing internal cells, and 

 at other times every frustule in the filament has presented the usual cha- 

 racter ; the present species will, I have no doubt, occur under similar circum- 

 stances, and thus spare us the necessity of doing violence to the law which 

 appears to regulate the self-dividing process throughout the Order. 



Supp. Plate LXI. 376. 



4. Odontidium mutabile, n. sp. Filament usually tenacious; V. 

 oval, elliptical, or linear-acuminate ; costse marginal, distinct, 20 in 

 •001". Length of frustule -0002" to -0014". v.v. 



Diatoma tenue, Kiitz. Bacill. xvii. 9 & 10? Fragilaria turgidida, F. bi- 

 punctata, et F. diophthalmia, Ehr. Inf. xv. 13?, 15?, 18? Odontidium 

 striolatum, Kiitz. Bacill. xxi. 20 ? 



Freshwater: frequent. Plumpton, Sussex, March 1852. Near Lewes, 

 April, June, and Nov. 1853, W. Sm. Peterhead and Lough Mourne Deposits. 

 Raasay Earth, &c. 



Exceedingly variable in size and outline of valve. This diversity has pro- 

 bably led to the formation of the various species noted in the synonymy 

 above, none of which, from the slight details given by Professor Kutzing, can 

 be confidently referred to the present species, and yet all of which may pro- 

 bably be varieties of this sportive form. O. mutabile ordinarily presents a 

 filament of considerable length, with the applied surfaces of the adjacent 

 valves adhering so firmly, that the valves of each frustule separate under the 

 action of an acid more readily than the cohering valves of contiguous 

 frustules. Still, this is so far from being the invariable condition of the 

 filament, that in some states of growth we find the frustules falling apart 

 almost as readily as in O. Tabellaria, or partially separating as in Diatoma, 

 thus giving occasion to those who regard their more or less tenacious con- 

 nexion as of essential value, to constitute several species out of a single form 

 in different stages of growth or decay. 



Plate XXXIV. 290. 



5. Odontidium Tabellaria, n. sp. Filament fragile ; V. oval-acu- 

 minate, or lanceolate; costae interrupted, delicate, 36 in *00l". 

 Length of frustule '0003" to -0009". v.v. 



/3. V. with a double inflation, v.s. 



Staurosira construens, Ehr. Microg. ? /3. Fragilaria binodis, Ehr. Microg. ? 



Fresh water. Plumpton, Sussex, April 1851, W. Sm. Ormesby, Norfolk, 

 Aug. 1853, Mr. Bridgeman. Horning, Norfolk, June 1854, Mr. Wigham. 

 /3. Ormesby, Mr. Bridgeman. Ben M c Dhui, Mr. P. Grant. Peterhead De- 

 posit. (Source of the Loiret near Orleans, June 1854, IV. Sm.) 



VOL. II. C 



