On Fterodina valvata. 25 



centre, and this suffices for all appliances illuminated from below. 

 But the necessary depth of the outer ring prevents the side beam 

 from falling on the prism, or the fingers from reaching its milled 

 head, when it is brought sufficiently close up. As it is scarcely 

 worth while to sacrifice the strength and durability of the stand to 

 suit a single accessory, it is best to bring the diatom-prism above 

 the stage. It is then completely unobstructed in its movements and 

 illumination, and works much the same with transparent objects as 

 Crouch's admirable parabolic reflector does with opaque ones. The 

 fitting required consists of a stage-plate sliding in the same dove- 

 tails as the ordinary object-carrier (which has then to be removed), 

 and carrying on four uprights, about two inches high, a thin plate 

 cut out in the middle with spring-clips to hold the object-slide. 

 This is of course carried, equally with the main stage, by all the 

 traversing and rotating movements. The diatom-prism, mounted 

 as usual, fits into the end of a tube carried by the sub-stage, and 

 long enough to project through the aperture of the principal stage. 

 The prism can thus, when desired, be racked-up close under the 

 object with every facility for adjusting and hghting. The bottom 

 plate is cut out on its lower side, so as to slide over this tube after 

 fixing the prism in the upper end. 



I may note for those who desire a cheaper and simpler, yet ser- 

 viceable, arrangement, that the diatom-prism works very well with 

 the excellent glass-stages of Messrs. Beck and Crouch, since these 

 are thinner than stages traversing and rotating mechanically can 

 well be made. As the rotation of these is satisfactory, and the 

 sliding movement sufficiently smooth and delicate for use with ^th 

 inch, they are adequate for ordinary prism- work. 



VI. — On Fterodina valvata: a Netv Species. 

 By C. T. Hudson, LL.D. 



Plate LXXII. 



Ptekodina is rarely met with in the neighbourhood of Bristol ; 

 and I had not taken more than six specimens, and these all of 

 P. patina, during the last sixteen years. Nothing can be more 

 tantalizing than capturing a solitary Fterodina. It is so deh- 

 cately transparent, that it is extremely difficult (indeed, I found 

 it almost impracticable) to see it with the naked eye well enough to 

 catch it with a pipette, in order to place it on the compressorium 

 in a drop of water, whose diameter should not greatly exceed the 

 creature's length ; and yet with solitary specimens this is the only 

 method by which a satisfactory investigation of its structm-e can be 

 secured. Even when this is accomplished, Fterodina is of such 



