156 THE VOYAGE OF H.M.S. CHALLENGER. 



This Diatom was obtained in the middle of the Pacific in a depth of 2900 fathom3. 

 As indicated in the outline adjoining the figure, it presents, when viewed from its zonal 

 side, a hat-like appearance. From the central areola, which is marked by a group of small 

 puncta, radiating lines of similar puncta proceed towards the periphery, although in some 

 cases they do not quite reach the circumference of the valve. The specific name has 

 reference to the form of the valve. 



Ooscinodiscus umbonatus, n. sp. (Plate II. fig. 8.) 



Valvis in centro depressis, hinc elevatis, dein depresso-complanatis ; puuctulis radiato- 

 fasciculatis ; fasciculi totidem denticulis ad marginem signantur. In mari Pacifico. 



This form, which is very closely allied to that last referred to, was found in the same 

 collection. Like the former, it possesses an umbonate form, but the centre of its elevated 

 part — as may be observed in the outline accompanying the figure — is notably low. The 

 valve is punctated in a radial manner, but the punctation differs from that of the preceding 

 in being fasciculate. The central area is ornamented with a small group of minute points in 

 the middle. At the circumference the fasciculi are separated by means of a short series of 

 small, closer, and more salient puncta. 



Ooscinodiscus (?) bifrons, n. sp. (Plate II. fig. 1.) 



Frustulum valvis dissimilibus ; striis egre conspicuis, et denticulorum lineis radianter 

 signatis, in una crebrioribus et ad marginem cessantibus, in altera rarioribus et marginem 

 attingentibus. In mari glaciali Antarctico. 



On examining a preparation made near the ice-barrier of the Antarctic on 24th February 

 1874, a small delicate disc was recognised. This is marked by irregular radiating lines, 

 which are sometimes interrupted by small but very salient puncta or denticules, and dis- 

 appear at a short distance from the circumference. By the use of strong oblique illumination 

 and the superior homogeneous immersion objective of Zeiss, the bottom of the valve is 

 found to be ornamented by striae of extraordinary deUcacy, which could only be adequately 

 represented in the figure by the use of a greater magnifying power. 



A second valve, also shown on Plate II. fig. 1, was found, which exactly corresponded 

 with the former in position and in size, but which presented a very distinct type of orna- 

 mentation, its surface being marked by sparsely disposed lines of granules or minute puncta. 

 These all originate much nearer the margin, and some go to the centre, while others disap- 

 pear sooner or later before reaching that point. It can hardly be doubted that, from the 

 perfect coincidence of the perimeters of the two valves, we have here to deal with a species 

 which, like Cocconeis, possesses dissimilar valves ; yet this supposition remains to be verified 

 by actual observation ; and it may, indeed, ultimately be discovered that they are the 

 representatives of distinct species. 



"Whether these valves belong to the genus Ooscinodiscus, or to another new genus, 



