674 Transactions of the Society. 



we are able to distinguish either valve even with imperfect sections. 

 The convexity of the dorsal valve fits nearly into the trough of the 

 ventral valve, so that they touch each other when of large size. 

 The section-bundle consisted of three frustules, of which the lowest 

 was probably near the period of its second division^ whilst the other 

 two had only recently emerged, therefore the lowest, compared with 

 the others, is probably backward in development. The girdle-band 

 comprises only the limit-line between the lowest and middle frustule. 

 With reference to the fine sculpture, mention must be made of a 

 portion of membrane hitherto unobserved, and which could not 

 have been well detected without transverse sections. This is the 

 projection on the edge of each valve, that is to say, at the limit 

 between valve and girdle-band, a spine turning far inwards as 

 shown in the figs, (r I), and is found fourfold in each frustule. All 

 sections of the two series prove clearly these four projections ; they 

 can only be the expression of a projecting line running along the 

 edge towards the inner space of the cell, I will call it edge-Hne, 

 which with the usual division of the frustule plays a prominent 

 part as we shall see. With low powers one sees only the four 

 small spines; applying the highest power one observes that the 

 larger spines, at any rate, are hook-shaped towards the valve-cavity, 

 fig. 43 ; it may actually become a hook, fig. 46. The end-valves, 

 being the oldest in the row, have the largest hooks and the strongest 

 edge-line. In the youngest valves one sees the partially developed 

 hooks, sometimes hardly nodule-shaped, fig. 44. 



The largest hooks penetrate 2 /j, into the cavity of the cell. 

 After having discovered the marginal lines in the sections it became 

 easy to trace them again in the surface view ; they are particularly 

 weU seen in balsam preparations (in consequence of the weaker 

 refraction of the walls). The older investigators, for instance, 

 Kiitzing (plate 20, IX., 1) and Ehrenberg, figure distinct dark 

 marginal lines ending with a nodule. The transversely cut marginal 

 line is in section far more conspicuous than the mid-ribs ; in most 

 instances one sees the mid-ribs as round nodules standing off the 

 membrane ; occasionally they may be seen somewhat more distinctly. 

 Altogether the dimensions are very small, and only in the most 

 favourable instances can one determine that they project inwards 

 and not outwards. The valve surfaces are always seen in good 

 sections as distinct rows of dots, pearl-like, and this is the expression 

 of the fine dots of which I spoke above. These pearls I declare 

 to be, according to the best and most reliable sections, chambers 

 closed on all sides situated within the membrane. The direction of 

 the cuts in the series in question being parallel with the actual 

 transverse strise, one can hardly expect to discover anything about 

 the condition of the latter. But if the section is very thin, one 

 sees on the inner side of the pearl row a fine straight line depressed 



