606 MICROSCOPIC FORMS OF VEGETABLE LIFE THALLOPHYTES 



neit by the genus Nitzschia, which is a somewhat aberrant form, dis- 

 tinguished by the presence of a prominent keel on each valve, divid- 

 ing it into two portions which are usually unequal, while the entire 

 valve is sometimes curved, as in N. sigmoidea, which has been used 

 as a test-object, but is not suitable for that purpose on account of 

 the extreme variability of its striation. Nearly allied to this is 

 the genus Bacillaria, so named from the elongated staff-like form of 

 its frustules ; its valves have a longitudinal punctated keel, and 

 their transverse striie are interrupted in the median line. The 

 principal species of this genus is the B. paradoxa, whose remarkable 

 movement has been already described. 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. 449. 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 Nitzschia and Bacillaria have been associated by Mr. Kalfs 



with some other genera 



A which agree with them 



in the bacillar or staff- 

 like form of the frus- 

 tules and in the presence 

 of a longitudinal keel, 

 in the sub-family A'itz- 

 schiece, which ranks as 

 a section of the Suri- 

 rellece. Another sub- 

 family, Synedrece, con- 

 sists of. the genus 

 Synedra and its allies, 

 in which the bacillar 

 form is retained, but 

 the keel is wanting, 

 and the valves are 

 but little broader than the front of the frustule. 



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 distinctive character of the genus 

 Surirella, in addition to the presence of the supposed ' canaliculi,' 

 is derived from the longitudinal line down the centre of each valve 

 (fig. 453, A) and the prolongation of the margins into * ala?.' 

 Numerous species are known, which are mostly of a somewhat ova in- 

 form, some being broader and others narrower than AS', constricta ; 

 the greater part of them are inhabitants of fresh or brackish water, 

 though some few are marine ; and several occur in those infusorial 

 earths which seem to have been deposited at the bottoms of lakes, 

 such as that of the Mourne Mountains in Ireland (fig. 468, b, c, X:). 

 In the genus Campylodiscus (fig. 454) the valves are so greatly 

 increased in breadth as to present almost the form of discs (A), and 

 at the same time have more or less of a peculiar twist or saddle- 

 shaped curvature (B). It is in this genus that the supposed ; cana- 



Fio. 453. Surirella constricta : A, side view 

 B, front view ; C, binary subdivision. 



