76 GfiEviLLE, on New Diatoms. 



Hah, Nottingham deposit, Maryland, U.S. 



Not rare, yet I cannot refer it to any described species: 

 It is a neat and brilliant little diatom. The puncta or minute 

 granules are rather distant, the largest being those imme- 

 diately external to the raised centre ; in the angles they 

 again become smaller. The angles themselves are frequently, 

 though not invariably, slightly dilated, as in fig. 7, and are 

 thickened in substance, but no distinct pseudo-nodule is 

 perceptible. 



Triceratium obscurwn, n. sp., Grev. — Small. Valve thin 

 and delicate, with nearly straignt sides and rounded angles ; 

 puncta equal, very minute, radiating in straight lines. Dis- 

 tance between the angles •0024". (Fig. 8.) 



Hab. South Naparima deposit, Trinidad. 



Contour exactly resembling that of T. condecorum, but the 

 radiating lines of puncta are perfectly straight. The puncta 

 are also somewhat more minute. 



Triceratium Harrisonianum, n. sp., Norman and Grev. — 

 Large. Valve with convex sides and slightly produced, 

 rounded angles ; pearly granules forming a marginal band of 

 radiating rows, and thinly scattered over the ample central 

 space, in which is a conspicuous network of large, elongated, 

 radiating cellules, sending down lines between the rows of 

 granules to the margin ; rows 4 in "OOl". Distance between 

 the angles -0070". (Fig. 9.) 



Hab. Barbadoes deposit (Springfield Estate); exceedingly 

 rare ; George Norman, Esq. 



A truly splendid diatom, belonging to a small, very natural 

 group, and, as is frequent in such cases, extremely difficult to 

 define satisfactorily. It may be, indeed, that most of them 

 constitute but one species ; and if so, it becomes all the more 

 necessary that they should be carefully figured and described. 

 This I hope to be able to do in a future series. T. mar- 

 garitacevm, described by Ealfs in the last edition of 'Pritchard's 

 Infusoria,^ is the only one hitherto published, and, as the first 

 known, may stand as the type. It is comparatively a small 

 species, the distance between the angles being only about 

 •0030", often less. All the members of the group, however, 

 possess the same structural arrangement, the central portion of 

 the valve being composed of large, radiating, elongated cellules, 

 which towards the margin become smaller and quadrangular, 

 each of the quadrangular cellules containing a round, pearly 

 granule. In none of the species are these characters seen so 

 conspicuously as in our new T. Harrisonianum. The outline 

 of the valve in these species varies considerably. According 

 to Ealfs, the sides of the valve in T. margarUaceum are straight 



