BRITISH DIATOMACE^. 21 



Genus 5. COCCONEIS, Ehr. 



Frustules adherent^ disciform ; valves elliptical, plane, with a median 

 line and central nodule. 



In this genus the frustules are closely adherent by the surface of 

 one valve to larger Algag, and are usually detached with difficulty. 

 The markings of the lower are less intense than those of the upper 

 surface, but in other respects the valves are symmetrical, and Mr. West 

 informs me that he has found the absence of a central nodule in the 

 lower valve, as noted by some writers, by no means a constant cha- 

 racter. 



1. Cocconeis Pediculus, JEJAr. F. arched ; V. elliptical, somewhat 

 angular ; disc delicately striated ; striae 40 in '00 1". Length '0008 

 to -0014". v.v. 



Ehr. Inf. xxi. 11. Kutz. Bacill. v. 9. 



Fresh water. Frequently attached to Cto/o/)^oya<//ome?'ff^a. Lewes, Aug. 

 1851 ; Wareham, Sept. 1851, W. Sm. Lough Mourne Deposit. Peterhead 

 Deposit, &c. 



Plate III. 31. Frustules with endochrome. Frontispiece fig. XXXI. 



The striae on this species are seen by a higher power as lines, concentric 

 with each extremity of the valve. 



2. Cocconeis Placentula, Ehr. F. plane, valve eUiptical, disc 

 striated. Length -0011" to -0016". v.v. 



Ehr. Inf. p. 194. Ehr. in Kiitz. Bacill. xxviii. 13. 



Freshwater: frequent. Wareham, July 1849, W. Sm. Lough Mourne 

 and Peterhead Deposits. Cantyre Peat and Dolgelly Earth. 



Plate III. 32. 



3. Cocconeis Thwaitesii, TV. Sm. F. arched; valve constricted 

 towards the obtuse extremities ; median line sigmoid ; disc faintly 

 striated ; striee 72 in -001". Length -0009" to -0013". v.v. 



Cymbella JlexeUa, Kiitz. Bacill. iv. 14. Achnauthidium flexellum, Breb. in 

 Klitz. Sp. Alg. p. 54. ad specim. authen. quse communicavit cl. De 

 Brebissou, Jan. 1853. 



Fresh water. Near Bristol, May 1848, Mr. Thwuites. Mountains in 



