304 



Bacillariece 



Genus Cymatopleura Turpin, 1827. The frustules are broadly 

 elliptical or sublinear in the valve-view, often broadly concave at 



each side and with subacuminate 

 apices. The valves are furnished 

 with a distinct but inconspicuous 

 pseudo-raphe and with very fine 

 transverse striations. Along the 

 margins are short costa? which 

 simulate coarse beads, and the 

 valve-faces are undulate. In the 

 girdle- view the frustules are linear 

 with a number of large undula- 

 tions along the lateral margins. 

 The genus is entirely freshwater 

 in habit. 



Fig. 144. A, Cymatopleura ellip- 

 tica (Breb.) W. Sm., from Comberton, 

 Cambridge ( x 350). B and C, C. 

 Solea (Breb.) W. Sm. ; B, girdle-view 

 of large specimen from Esholt, W. 

 Yorks. ; C, valve-view of small speci- 

 men from Cornwall ( x 350). 



C. Solea (Breb.) W. Sm. is the com- 

 monest British species ; the valves are 

 elongated and slightly narrowed in the 

 median portion; length 50 130/z; 

 fig. 144 B and C. C. elliptica (Breb.) 



W. Sm. is almost equally abundant 

 and possesses broadly elliptical or elliptic-lanceolate valves; length 80 140 /*; 



fig. 144 A. 



Genus Surirella Turpin, 1827. [Original spelling ' Suriraya.'] 

 In the valve-view the frustules are elliptical, linear, ovate, or 

 sometimes twisted. There is a median pseudo-raphe and strong 

 transverse costa3. The pseudo-raphe of one valve is parallel to 

 that of the other. The valves possess a sagittal keel and short 

 wing-like projections (alse) along their margins. The four sets of 

 ala3 are best seen in a transverse section. The girdle- view is 

 subrectangular, oblong, or cuneate, and the girdle exhibits irregular 

 longitudinal striations. 



S. biseriata Breb. (length of valves 100 170 p.; fig. 145 A) and . robusta 

 Ehrenb. (length of valves 160 230 p.) are two of the largest and most 

 frequent freshwater species. A variety of the latter, S, robusta var. splendida 

 (Ehrenb.) V. H., is general in the British freshwater plankton (fig. 145 C). 

 A much smaller species, A', ovalis Brc'b., with numerous varieties, is very 

 abundant ; length of valves 16 80 /z. S. spiralis Kiitz. is remarkable for the 

 twisting of the valves round the longitudinal axis. 



