1884.] 



MICROSCOPICAL JOURNAL. 



47 



which we had glimpsed before are a 

 regular system covering the whole 

 surface, and radiating in straight lines 

 from the centre of the valve to the 

 sides of the triangle. This inner film, 

 therefore, if it were separated from 

 the honeycomb behind it, would be a 

 shell of itself, marked somewhat like 

 T'rifiacria Regina^ but much finer. 

 The dark hexagonal lines show the 

 attachment to the deep hexagonal 

 walls of the areolae within, and the 

 dark spots upon the upper surface 

 of the valve show that the upper ends 

 of the areolae are dome-shaped within. 

 A valve inclined at an angle to the 

 light will show that these spots 

 are not holes in the upper surface, 

 but that the silex film is continuous 

 and evenly curved with the general 

 contour of the shell. 



Returning now to a balsam mount, 

 and using a high power with trans- 

 mitted light, it will be found that in 

 the case of a valve with the concave 

 side toward the eye, after adjusting 

 the lens carefully upon the finely 

 dotted film, we may lower the tube 

 and find the " eye-spot " in a plane 

 considerable lower. Or if we choose 

 a valve with the convex side upward, 

 after focussing sharply upon the edge 

 of the hexagons, we shall have to 

 go deeper to bring the "dots" into 

 focus. In the last case the upper 

 lamina of the shell, being without dis- 

 tinguishing marks, is not visible by 

 transmitted light. 



We have thus been led to the con- 

 clusion that the Triceratium is 

 formed of two laminte connected by 

 a hexagonal net-work, of which the 

 areolse are about as deep as the diame- 

 ter of the hexagons ; that the inner of 

 these laminiE is finely dotted with 

 lines of punctae radiant from the cen- 

 tre of the triangle, and that the outer 

 lamina is very thin over the centre of 

 each hexagon, to which it is firmly 

 connected by the walls of the areolae, 

 which are thickened so as to give a 

 hemispherical interior form to the 

 upper end of each. 



Eupodiscus argus. — The structure 



oi Eupodiscus argus was at one time 

 a good deal discussed, and Mr. Stod- 

 der gave what seems to be nearly the 

 true description of this shell. The 

 opaque Cuxhaven slide is full of ex- 

 cellent examples of it, and the quar- 

 ter-inch objective suffices to determine 

 its form beyond dispvite. The convex 

 surface is found to be deeply pitted 

 with irregular areolae having very 

 thick walls, looking very much like 

 the exterior of a peach stone in the 

 character of the depressions and 

 their relative size when compared to 

 the thick walls around them. No 

 shifting of light gives any trace of 

 a film covering these areolae on this 

 side ; but they retain the dead-white 

 color in whatever direction they are 

 seen. Broken fragments found here 

 and there show the section of the 

 areolae, with the boldly-projecting 

 and heavy net-work of walls. The 

 processes stand out like horns of the 

 clearest glass. The concave surface 

 presents a striking contrast to this. 

 It is smooth and well glazed, but on 

 carefully manipulating the light we 

 find in this, as on the inner surface of 

 the Triceratiutn^ a system of dots in 

 radiating lines, resembling a good 

 deal the marking of Actinocyclus 

 Ralfsii^ a shell which is also abund- 

 ant in this gathering. On the inner 

 surface of the Eupodiscus will also 

 be found indentations marking the 

 bases of the processes and fitting into 

 them. 



In examining transparent slides of 

 the same material, I have occasion- 

 ally found a shell of E. argus in 

 which the heavy net-work had been 

 partly removed by accident, and the 

 semi-opaque and characteristic ap- 

 pearance which the valve ordinarily 

 has was in contrast, side by side, with 

 the interior film, as if the specimen 

 had been artificially prepared for the 

 demonstration. The arcolation varies 

 in different shells. In some it is al- 

 most as regular in pattern as some 

 examples of Coscinodiscus radiatus. 

 In others it is hard to see any ap- 

 proach to regularity of arrangement, 



