NEW FORMS OF MARINE DIATOM ACE^. 501 



the central portion being almost or quite hemispherical. It is so convex, that, 

 when the marginal zone is in focus, the middle part appears as if full of air. Cells 

 in lines, generally radiate, rather small, irregular in outline, and unequal in size to 

 some extent. As the rays diverge from each other towards the margin, the space 

 is often filled up by a bifurcation of the rays, which gives an aspect of irregularity 

 to the markings. 



This fine disc occurs in one of the Lamlash Bay dredgings, in which, however, 

 it is very scarce indeed. We must hope that it will be found in greater abun- 

 dance. The broad marginal zone or brim, and the very convex middle part, give 

 to it a great resemblance in shape to a " wide awake" hat ; but I have named it 

 from its resemblance to a shield with a large boss in the centre. 



49. Coscinodiscus centralis, Ehr. PI. XL, fig. 49. Diameter of disc 0-004" to 

 0-009", or even 0-01". Surface regularly cellulate. Cells rather small, hexagonal, 

 equal, except at the centre, where there are three large oblong cells, meeting in 

 a point ; and between these, a little farther from the centre, three more cells, 

 a little smaller. The valve is remarkably transparent, and from this, and the 

 small size of the cells, it is apt to be overlooked, although, when accurately 

 focussed, the cells are very distinct. It has frequently a yellow or straw colour, 

 in balsam, under the ^-inch objective. 



This very beautiful disc is by no means rare in the Glenshira Sand ; but when 

 I described that deposit, I was not well acquainted with the discs figured by 

 Ehrenberg, and supposed it to be a form of C. radiatus, or else C. concinnus. It 

 agrees exactly with Ehrenberg's figure of C. centralis. It occurs not unfrequently, 

 both in Lamlash Bay and in Loch Fine, and it reaches occasionally a diameter 

 exceeding that above mentioned. 



50. Eupodiscus subtilis, n. sp. (Ralfs.) PI. XL, fig. 50. Diameter of disc 

 0-0033". Surface apparently convex, very hyaline, and very densely marked with 

 fine lines, and indications of minute cells, of which the lines are probably com- 

 posed. In the centre is a rather large circular spot, and the usual pseudo-nodule 

 of the genus is placed close to the margin. 



This disc was first noticed by Mr Ralfs in the early part of this winter 

 (1856-57). He distributed specimens as probably Coscinodiscus concinnus; but 

 when Dr Gtreville and myself came to examine it with object-glasses of high 

 power and of superior quality, it was soon recognised as an Eupodiscus. This, 

 so far as I know, was first done by Dr Gtreville. I had frequently observed a 

 disc like it, with very delicate and obscure markings, in some of my dredgings ; 

 but it was Dr Greville also who first ascertained that these discs, which I had 

 taken for granted were Coscinodiscus concinnus, were really identical with the 

 disc of Mr Ralfs. As the form occurs in these dredgings, therefore, though less 

 abundantly than in Mr Ralfs' gathering, I figure it here as an Eupodiscus. 



51. Campylodiscus centralis, n. sp. PL XL, fig. 51. Form orbicular, or nearly so. 



VOL. XXI. PART IV. 0* T 



