124 THE VOYAGE OF H. M.S. CHALLENGER 



augentur numero ; corona viginti-quatuor denticulorum in marginali limbo arctissime 

 striate In oceano Pacifico. 



The centre of this magnificent disc presents a small, smooth, irregular area, to which 

 radiating lines of equal round granules converge on all sides. Some of these lines become 

 interrupted as they pass inwards, owing to the diminution of the size of the space, and 

 where such interruptions occur, long triangular spaces become interpolated. Near the 

 margin the granules suddenly diminish in size, and assume the appearance of very minute 

 radiating striae. Twenty-four denticules project around the broad border. Although this 

 Diatom does not seem to differ much from the two preceding, the small irregular umbilical 

 area, and the notable convexity of the valve, point to its true specific character. In 

 addition to the figure of the valve from the valval aspect a small ideal figure from the 

 zonal side has been added in order to show the notable convexity of this frustule. 



Cestodiscus coronatus, n. sp. (Plate VII. fig. 9.) 



Disciformis, medio granulatus, ad marginem striatus ; centrum a margine dividitur per 

 coronam e cellulis oblongis, grandiusculis, a qua exterius oritur denticulorum series. In 

 mari Pacifico. 



In this Diatom, which also belongs to the group having a continuous system of granu- 

 lation and a small umbilical area, the beautiful granulation radiates from a small umbilical 

 area in the central part of the valve, while the margin is ornamented with delicate striae. 

 The two areas, which are thus differently sculptured, are separated by means of a corona 

 of beautiful large oblong cellules, from which a series of spines or denticules pass towards 

 the circumference. Apart from these denticules the other structural details recall those of 

 the genus Brightwellia, Ralfs. 1 



Cestodiscus gemmifer, n. sp. (Plate II. fig. 7.) 



Disciformis ; granulis rotundis ; grandiusculis ; circumconfertis, radiantibus, ad aream 

 centralem raris et irregulariter distributis, aequalibus, ad marginem minuentibus et 

 adspectum zonae marginali striolatum tribuentibus ; spinae vel denticuli plures (38) mar- 

 sinales. In oceano Pacifico. 



This large form has a disc about -fa of a millimeter in diameter. It is ornamented by 

 large round granules, which are crowded towards the peripherj r , but rare and scattered 

 about the centre. These granules gradually diminish in size near the marginal zone, and 

 finally assume the appearance of fine radiating striae. The spines which project around the 

 margin are more numerous than usual, being thirty-eight in number in the specimen figured. 

 The peculiar irregular distribution of the large granules in the middle area, and the laro-e 

 smooth central space are, however, the chief distinguishing features of the present frustule. 



1 Micr. Journ., n. s., vol. i., 1861, p. 74. 



