REPORT ON THE DIATOM ACEjE. 123 



occurring between two of the radiating lines of granules which proceed from the sub- 

 marginal processes and run towards the centre. 



Greville defines the genus Cestodiscus] as follows : — "Frustules disciform (circular or 

 oval), disc with radiating granules or cellules, and a submarginal circle of obtuse processes 

 unconnected by means of special radiating lines of cellules with the centre." 



To this description the following character, which may be constantly observed in all 

 specific forms, should be added, namely, the radiating granules or cellules of the central 

 part of the disc become more or less abruptly minute crowded granules or points, on 

 approaching the margin. This may be well seen in Cestodiscus trocos described below. 



Cestodiscus trochus, n. sp. (Plate VII. fig. 1.) 



Valvis rotundis ; distinctorum granulorum radiis a centro dimanantibus, pluribus brevi- 

 oribus insertis ; granulis rotundis, distinctis, sequalibus, ad marginem sensim diminuentibus 

 et stipatis ; ad extremum plurium (32) denticulorum extat corona. In oceano Pacifico. 



In this beautiful disciform Diatom there is a group of small, somewhat sparsely dis- 

 posed granules in the centre, and from this region a series of distinct radiating lines of 

 granules pass out towards the periphery near to which other shorter lines are interposed 

 between these. The granules are usually large and equal, but become, smaller near the 

 margin, and finally in a narrow marginal belt become extremely fine and crowded so as to 

 present the appearance of very delicate striae. Around the striated border very small 

 processes occur. These are irregularly disposed, and accordingly divide the circumference 

 irregularly. They are thirty-two in number in the specimen figured. The specific name ~ 

 has reference to the wheel-like appearance presented by the larger radiating granules 

 that form lines across the valves. 



Cestodiscus trochus, n. sp., var. no v. (Plate VII. fig. 3.) 



A variety of the preceding species is here shown. It differs from the typical form (1.) 

 in the size of the granules, which are larger ; (2.) in the condition of the centre, which is 

 marked by a small irregular group of somewhat smaller granules; (3.) in the smaller 

 number — twenty-four — of denticules in the submarginal corona; and (4.) in the more 

 abrupt change in the size of the granules at the border, where they assume the 

 appearance of minute strise. The general aspect of the two frustules, however, is the same, 

 and the above distinctions cannot be looked upon as of specific importance. 



Cestodiscus a con vexus, n. sp. (Plate VII. fig. 6.) 



Forma rotunda, convexa ; grandiusculorum granulorum ordinibus ab umbilicali area 

 dimanantibus, et continuo crebrescentibus ; ad marginem granuli minuuntur mensura, 



1 Trans. Mier. Soc. Lond. n. s., vol xiii., 18G5, p. 48. * '■fXo'f, a wheel. 



