The Rev. W. Smith on the Diatomacese. 5 



employed by Mr. Ralfs and other English writers, and using the 

 term " front \iew " to denote the aspect of the frustule when the 

 connecting membrane and valvular suture are turned towards the 

 observer, the words " lateral " or " side view " being employed 

 when the general surface of one of the valves is directed to the eye. 



I. have employed in my investigations a microscope constructed 

 by Messrs. Smith and Beck, with a ;^-inch object-glass, and eye- 

 piece, giving together a power of 400 diameters. In some cases I 

 have availed myself of an excellent ^th object-glass manufactured 

 by Mr. A. Ross, giving with the above eye-piece a power of 

 880 diameters ; and I have latterly employed Wenham's Parabolic 

 Reflector, a new and ingenious instrument for seeming a very 

 oblique illumination of the object, supplied by Messrs. Smith and 

 Beck, and which has revealed markings and aided in the deter- 

 mination of forms respecting which I should otherwise have felt 

 a doubt ; but when I have employed either of the latter means of 

 investigation I have not failed to mention the fact, and the de- 

 scriptions are otherAvise to be regarded as depending upon a 

 power of 400 diameters, the figures given being all drawn to this 

 scale. 



The frustules examined have been prepared either by exposure 

 to a strong heat on talc, or by maceration in nitric acid. It is 

 only after such preparation that the form and markings of the 

 siliceous valves can be accurately determined, or the frustules 

 satisfactoi'ily mounted in Canada balsam for permanent observa- 

 tion. I have however noted by the letters v. v. appended to the 

 specific descriptions, the circumstance of my having examined 

 the frustules in a living state, and by v. s. when I have seen 

 only the desiccated or prepared valves. 



The fii'st genus, the British species of which I proceed to de- 

 scribe, may be defined as follows : — 



Campylodiscus, Ehr. 



Valves equidistant (not concave). Frustules free, solitary, or 

 when undergoing self-division, in pairs, disciform, saddle- 

 shaped. 



The species included under this genus may all be recognized 

 by the characteristic bend or contortion of their surfaces, which 

 gives to the frustule under certain aspects the semblance of a 

 miniature saddle. Kiitzing has indeed removed from Campylo- 

 discus and placed in Surirella, several species possessing this cha- 

 racter, apparently for no other reasons than that the striae or costae 

 are confined to the margins of the valves and are parallel, not 

 radiate. When we consider that the strise are often exceedingly 

 difiicult of detection, and that their direction merely cannot be 



