122 THE VOYAGE OF H.M.S. CHALLENGER 



Stictodiscus trigonus, n. sp. (Plate XIII. fig. 1.) 



Forma triangularis, apicibus obtuso-rotundatis, lateribus tumidufis ; granuli medio 

 irregulares et rariores, ad latera lineis per plicas separatis, ad apices punctuli densiores. 

 In mari Japonico. 



The slightly tumid sides, and obtuse, rounded angles of this form are noteworthy. 

 The granules are few, and irregularly placed in the centre, but are more abundant and 

 arranged in lines divided by folds at the sides, while they are smaller and still more 

 numerous at the extremities. The gradually decreasing size of the granules from the 

 centre towards the periphery is a well-marked characteristic of this Diatom. 



Stictodiscus hexagonus, n. sp. (Plate XVII. fig. 17.) 



Valvis hexagonis, granulatis ; apicibus late rotundatis ; margine granulorum ordine 

 terminato ; granulis grandiusculis circum subregularibus, medio nonnullis ; superficies 

 medio plicata. In Atlantico septentrionali. 



The present curious hexagonal frustule has very rounded angles, and a line of small 

 round granules at the extreme border. The convex surface is flattened at the margin, and 

 is ornamented medially by large granules. These occur in sparsely disposed subregular 

 lines at the periphery, but are almost entirely absent in the centre, where the surface is 

 reticulate, the meshes of the network being very variable in size and irregular in form. At 

 the angles a few closer lines of granules are to be observed. These are disposed in a 

 fan-shaped manner, as in all the polygonal forms of the genus. An intra-marginal thin 

 bine, almost parallel throughout to the edges of the frustule, forms a well-marked boundary 

 line between the peripheral and intermediate areas, on the latter of which a reticulation 

 such as is found at the centre is not manifest. 



Stictodiscus hexagonus, n. sp., var. nov. (Plate XVII. fig. 14.) 



A variety of the last-mentioned type is here represented. It differs from the typical 

 frustule in the smaller size of the disc and granules, in the relatively larger size of the 

 central network, and in the total absence of those lines of small granules that constitute 

 the margin of the valve of Stictodiscus hexagonus. Both the frustules were found in the 

 South Atlantic. 



Cestodiscus, Grev. 



This genus was instituted by Greville 1 in 1865 tojnclude some very elegant discoidal 

 frustules, which differed from Eupodiscus only in possessing numerous marginal processes 

 instead of a few. It greatly resembles the genus Aulacodiscus, and might, indeed, be 

 identified with it if the intra-marginal shortly tubular processes of the latter were not 

 connected with the centre by means of a distinct furrow formed by a well-defined interval 



1 Trans. Micr. Soc. Land., n. e., vol. xiii., p. 2. 



