296 A. A. MERLIN ON THE MINUTE STRUCTURE OF 



which ten have been counted within the space of one areolation. 

 These latter secondaries are arranged in a totally difterent manner 

 to those of the upper domed cap, and would probably be alone 

 visible with a dry lens. This is the only example I have yet 

 noticed of areolations crowned with double plates or caps, closely 

 superposed, each bearing secondaries of a distinctly different type. 



The next point to which I venture to call your attention is the 

 existence of very fine structure on the upper surface of some 

 Navicida prcetexta valves, apparently extending over the entire 

 area of the valve, when the primaries are held in distinct " black 

 dot " focus (Fig. 3). I have worked on several occasions for 

 hours together at these specimens, and have been unable to 

 satisfy myself as to whether this appearance, which is almost at 

 the limit of my vision, should be attributed to a surface roughen- 

 ing of the silex, analogous to that of ground glass, or to the 

 presence of regular structure of surpassing minuteness. I have 

 noticed a similar structural appearance on the surface of the 

 hyaline bands of a Navicida lyra on the same slide (Fig. 4), It 

 has been searched for on other forms, but without success. 



Several valves of Pleurosigmaformoswm and one of P. angidatuin 

 have many of their so-called " markings " damaged and choked 

 up, the individual '* dots " being in numerous instances only partly 

 jEilled in, while others adjoining are normally round. The broken 

 edge of a P.formosum (Fig. 5) exhibits a projecting line of five 

 " black dots," the first of which has been halved by the fracture, 

 the second and third left entire, and the fourth and fifth partly 

 carried away. 



A fine specimen of Aidiscits, apparently identical in structure 

 with the A. Oamaritensis figured in our Journal for January 

 1887, has large processes with difficult secondaries of the xl. scidjjtns 

 type (Fig. 6). 



Lastly, two Orthoneis valves, resembling in outline and size 

 the 0. splendida, have long rod-shaped primaries running at right 

 angles to the raphe, each primary being shghtly constricted in 

 the centre, where it is bridged by a broad silicious bar (Fig. 7). 



Although all the above are stated to be recent marine forms, 

 the curiously weathered and dilapidated appearance of the structure 

 on the P. formosum and P. augidatimi valves in this preparation 

 closely resembles that of a fossil P, formosum from Sendai, Japan, 

 in my possession. 



