500 PROFESSOR GREGORY ON 



wards the margin, but somewhat confused towards the centre. Puncta or granules 

 larger towards the centre than at the margin. Aspect of valve glassy, puncta 

 nitescent, very much as in Cocconeis nitida (fig. 26). 



This pretty disc was figured without a name, from an imperfect specimen, in 

 my last paper on the Glenshira Sand [Trans. Mic. Soc, vol. v., pi. i., fig. 50). 

 Having found it tolerably frequent in Lamlash Bay, I now figure a perfect exam- 

 ple, which, provisionally, I refer to Coscinodiscus. 



46. Coscinodiscus punctidatus, n. sp. PI. X., fig. 46. Diameter of disc 0003" 

 to 00036". It is marked by very fine and obscure lines, which, near the margin, 

 are traceable as rays, but which soon become fainter, and apparently wavy at the 

 same time, as they proceed towards the centre. Over the whole surface are 

 scattered, sparsely, small puncta, which, in a certain focus, appear as points of 

 light. 



This disc was also figured, but not named, in my last paper on the Glenshira 

 Sand (Trans. Mic. Soc, vol. v., pi. i., fig. 48); but the specimen here figured is 

 a better one, and shows more of the very obscure structure. It is impossible, in 

 the present state of our knowledge of it, to refer it with certainty to any genus ; 

 but it may be a Coscinodiscus. It occurs in Lamlash Bay and in Loch Fine, but 

 is not very frequent. It may possibly prove to be the end view, or the dissepi- 

 ment, of a Melosira or an Orthosira. 



47. Coscinodiscus concavus, Ehr. PI. X., fig. 47. Diameter of disc 0-0025" to 

 0-0043". Margin entire, transversely striated. Stride about 10 in 0-001". Surface 

 concave, covered with large, equal, hexagonal cells, arranged as in a honey- 

 comb. 



This beautiful disc occurs rather sparingly both in Lamlash Bay and in Loch 

 Fine. It agrees exactly with one of Ehrenberg's figures of C. concavus. But in my 

 last paper on the Glenshira Sand, I have figured another disc {Trans. Mic. Soc, 

 vol. v., pi. i., fig. 52), which differs from the present form in having a punctum 

 in the centre of each cell, and in the margin being formed of very large cells, 

 divided by strong bars, which appear to project from the plane of the valve, and 

 which also extend beyond the outer margin. This disc I at first suspected to 

 belong to some of the Polycystineae ; but I afterwards found it figured by Ehren- 

 berg as C. concavus. I confess that I cannot believe these two discs to be of one 

 species ; but that represented in fig. 47 seems to be a true Coscinodiscus ; and as 

 it is identical with one of Ehrenberg's examples, we may consider it as the true 

 C. concavus, leaving the other for farther investigation. (I have recently found, 

 in some of the Clyde dredgings the other disc, just alluded to as having been 

 figured in my paper on the Glenshira Sand, and as being also named by Ehren- 

 berg, C. concavus.) 



48. Coscinodiscus umbonatus, n. sp. PI. X., fig. 48. Diameter of disc about 

 0;0045". Surface densely cellulate, having a broad, nearly flat, marginal zone, 



