114 THE VOYAGE OE H.M.S. CHALLENGES. 



or polygonal form, prevented him from recognising affinities between two groups of forms 

 so distinct from one another, and although from the disposition of the granules, in lines 

 radiating from the sides and sparsely disseminated in the centre, he thought of constituting 

 a special group for Triceratium harrisonianum and Triceratium margaritaceum, Ralfs, he 

 was unaware of the importance of the folded character of the former, which, taken in 

 conjunction with the special distribution of the granules, should have induced him to class 

 it with Stictodiscus. 



Again, Greville 1 determined as Triceratium irijlatum, a three-sided form furnished 

 with lines or veins radiating from the margin to the middle of the radii, and possessing 

 puncta or granules which were large and sparsely disposed in the central part, but smaller 

 and more numerous on the circumference. These characters, however, are precisely those 

 of the genus Stictodiscus, so that the organism should be named Stictodiscus infiatus. 



The name Triceratium lineolatum, Grev., 2 cannot, however, be corrected in the same 

 manner, because though in this veins or folds occur on the plane of the valve at its 

 margin, the valve is elsewhere covered throughout with equal, minute, radiating granules 

 — a character which I have never met with in any of the different specific forms of 

 Stictodiscus. 



That, on the other hand, Triccratium parallelum, Grev., 3 is a true Stictodiscus 

 cannot be doubted. It possesses four or six angles, its centre is areolated by furrows or 

 folds which are distributed in a retiform manner, and its border is furnished with granules 

 disposed in radiating lines. It should therefore be named Stictodiscus parallelus. 



To the same genus should also be ascribed Triceratium quadratum, Grev. (loc. cit:, 

 fig. 19), because it shows short folds at the circumference, and the surface is covered with 

 radiating lines of granules, which however are few, and scattered in the central space. It 

 may also be observed that Greville reports having seen specimens with a sort of umbilicus, 

 or, at least, a somewhat irregular circle of smaller cellules, around which the ordinary 

 cellules are often more or less scattered before they pass into radiating lines — a circumstance 

 which would seem to indicate that the frustule in question is a coronated Stictodiscus. 



In the case of Triccratium polygonium, Grev., 4 we have to deal with a true Stictodiscus. 

 This beautiful Diatom, with its six straight sides and obtuse angles, not only possesses 

 elegant lines of radiating granules except in the centre, which is slightly reticulate, but, as 

 shown in the figure, as many depressed lines or folds as exist in the recognised forms of 

 Stictodiscus occur at the margin. The name should accordingly be changed to Stictodiscus 

 polygonius. 



The Triceratium quadricorne of Greville 5 is also probably a Stictodiscus, as its surface 



1 Micr. Journ., n. s., voL iii. p. 232, pi. x. fig. 15. 



2 Micr. Journ., n. s., vol. iii. p. 233, pi. x. fig. 16. 



3 Micr. Journ., n. s., vol. v. p. 104, pi. ix. figs. 22 and 23. 



4 Micr. Journ., n. s., vol. v. p. 105, pi. viii. fig. 14. 



5 Micr. Journ., n. s., vol v. p. 103, pi. ix. fig. 16. 



