294 



DIATOMACE.E '. FRAGILLARIEjE ; SYNEDRE^E. 



already described (§ 221). Owing to this displacement of the 

 frustules, its filaments seldom present themselves with straight 

 parallel sides, but nearly always in forms more or less oblique, 

 such as those represented in Fig. 125. This curious object is an 

 inhabitant of salt or of brackish water. Many of the species 

 formerly ranked under this genus are now referred to the genus 

 Diatoma. The Genera Nilzschia and Bacillaria are now asso- 

 ciated by Mr. Ralfs,* with some other genera which agree with 

 them in the bacillar or staff-like form of the frustules and in the 

 presence of a longitudinal keel, in the Sub-family Nitzschiece, 

 which ranks as a section of the Surirellece. — Another Sub-family, 

 Synedrece, consists of the genus Synedra and its allies, in which 

 the bacillar form is retained (Fig. 145, I), but the keel is wanting, 

 and the valves are but little broader than the front of the frus- 

 tule. 



Fig. 129. 



Surirella constHcta :— a, side view; b, front view ; c, binary subdivision. 



225. In the Surirellece proper, the frustules are no longer 

 bacillar, and the breadth of the valves is usually (though not 

 always) greater than the front view. The Genus Surirella (Fig. 129) 

 is one of those in which the supposed ' canalicular system ' of 

 Prof. W. Smith is most strongly marked ; it is not, however, t>y 

 any means equally conspicuous in all the species. The distinctive 

 character of this genus, in addition to the presence of the ' canali- 

 culi,' is derived from the longitudinal line down the centre of 

 each valve (a), and the prolongation of the margins into 'ahe.' 



* See Pritchard's " Infusoria," 4th Ed. p. 940. The genus NitzscJiia 

 was in the first instance placed by Mr. Ralfs in the family Fragillariece, 

 and the genus Bacillaria in the family Suvirellea. 



