12 The Rev. W. Smith on the Diatomacese. 



rounded ; costse marginal. Average length of valve xroo ^^ 

 an inch, greatest breadth of ditto 2 j^o of an inch {v. v.). 

 S. ovata, Kiitz. Bacill. p. 62. tab, 7. figs. 1, 2, 3, 4. 



In streams. Near Corfe Castle, Dec. 1849. Lewes, 1850. 



Swirella salina, mihi. Frustule on front view wedge-shaped, on 

 side view ovate ; the larger end rounded and the smaller more 

 or less pointed ; costse marginal. Average length of valve ^^q 

 of an inch, greatest breadth of ditto -yt^-qq of an inch [v. v.). 

 In salt-water ditches. Poole Bay, 1847. 



The three preceding species are closely allied, and have little 

 to distinguish them except their size and habitats. The first two 

 are found in fresh water, and of these S. ovalis is much the 

 larger, has the extremities of the valves less distinctly rounded, 

 and presents a stouter and more oblong aspect on the front view. 

 The last, S. salina, is a salt-water species, in form closely resem- 

 bling S. minuta, but usually larger, more distinctly ovate, and 

 with the smaller extremity of the valve in most of the frustules 

 somewhat attenuated. 



Plate III. fig. 5. Front and lateral view of S. minuta ; fig. 6. front and 

 lateral view of S. salina ; fig. 7. front and lateral view of S. ovalis. 



Cymatopleura (nov. gen.), mihi. 



Valves undulated, margins not produced into alse. Frustules free, 

 solitary, or when undergoing self- division, in pairs. 



I find it impossible to refer the species I am about to describe 

 to Surirella, with which genus the two first have been united by 

 Kiitzing and others. The undulated surface of the valves seems 

 to indicate a peculiarity of structure sufficient to constitute a 

 generic difiierence, and the absence of alge and costse implies a 

 further diversity in the internal character which cannot be re- 

 garded as unimportant. I should have been glad to have adopted 

 Mr. Hassall's genus " Sphinctocystis," but as this term refers 

 merely to a peculiarity in the external form of one of the species, 

 I am obliged to reject it also. 



Cymatopleura solea, mihi. Frustule on front view oblong, linear ; 

 side view fiddle-shaped, symmetrically divided by a central 

 sinus on each margin ; surface of the valve with about six un- 

 dulations, striated, with a smooth central line. Length of 

 valve from ji^ to yi^ of an inch, breadth of valve in older 

 specimens about y^^th of the length [v. v). 



Navicula librilis, Ehr. Surirella solea, Kiitz. Bacill. p. 60. tab. 3. 

 fig. 61 ; Phy. Ger. p. 71 ; Sp. Alg. p. 34. Sphinctocystis librilis, 

 Hassall, Brit. Freshwater Algse, p. 436. pi. 102. fig. 3. 

 Common in ditches and ponds generally mixed with Oscillatoriece. 



