66 THE VOYAGE OF H.M.S. CHALLENGER. 



related to the Campylodiscus tcallichianus of Greville ; 1 but it differs from the latter in its 

 considerably larger size, its far greater number of canaliculi, and in the arrangement of its 

 spines, which, in the Grevillean species, appear in the middle part of the lines only, and 

 not throughout their whole lenjrth as in the form from Tahiti. Moreover, the central area 

 of Campylodiscus uxdlichianus is linear, and is sharply defined round its entire margin, 

 and not at the two extremities only as in the present case. 



These points of distinction cannot, however, be regarded as essential, but depend 

 merely on the greater or less number of certain details of structure in the two types, so 

 that the Diatom now in question has been indicated merely as a variety of the Grevillean 

 frustule. 



Campylodiscus anceps, n. sp. (Plate XVI. fig. 2.) 



Valvis rotundis, vix curvatis ; e maximis ; triplici granulorum circulo bis opposite inter- 

 rupto, a quo plurimse (64) brevissimse costulse dimanant. Ad Zebu in mari Philippinarum. 



This is the most singular and novel form of Campylodiscus which has to be recorded. 

 It possesses a large and almost perfectly spherical disc, which is bounded by a triple 

 granulated margin, from which there proceed centripetally numerous short costules. At 

 two diametrically opposite points the marginal bands are interrupted, and at these points 

 small embryonic costules occur. By means of oblique illumination it may be seen that 

 the internal border of the rim is very finely striated, while the centre is devoid of 

 ornamentation of any kind. It is also noteworthy that, contrary to what occurs in all 

 other known species of the genus, the valve is almost smooth. It differs too from the 

 other species by the absence of marginal wings ; but the non-existence of these is explained 

 by the all but entire absence of a valval curvature. 



This curious frustule might be regarded as presenting a transition to the genus 

 Coscinodiscus, were it not for the diametrically opposite areolaa that occur upon its 

 rim, where augmentation takes place, and in this respect it approaches nearer to the genus 

 Surirdla. 



The diameter of this novel form is 170 n- 



Nitzschia, Hassall, W. Sm. 



The genus Nitzschia, which was instituted by Hassall in 1845 in memory of one 

 of the first observers who called the attention of naturalists to the great family of the 

 Diatomacea?, embraces the free compressed more or less bacillar frustules, whose linear 

 keeled valves are provided with one or more longitudinal lines of puncta, the keel being 

 often excentric. This last character is sufficient to distinguish it from the genus Amphi- 



1 This frustule is figured in' Schmidt's Atlas, pi. xiv. figs 15. and 16. 



