30 THE VOYAGE OF H.M.S. CHALLENGER 



particular, the shape of the apex is constant in each type. As in the variety last 

 mentioned there are two smooth longitudinal areas, but these are slightly constricted in 

 the middle, where they are ornamented by a few prominent rounded dots. 

 The specific name is taken from the character of the apex. 



Navicula bullata, Norman, var. rhomboidea, nov. (Plate XXX. fig. 7.) 



This bullate navicular frustule is subrhomboidal in shape, and the prolonged apices of 

 the valves are rounded. The valve is plicated by several longitudinal furrows, but, although 

 this characteristic is exceedingly distinct, the form can be regarded only as an interesting 

 variety of Navicula bullata. As in the previous varieties there are a few well-marked 

 dots at the centre of the smooth longitudinal areas which are here plano-convex in outline 

 and situated around the central area of the valves. This specimen was gathered at Zebu. 



Navicula brasiliensis, Grun. (Plate XX. figs. 1 and 3.) 



We have here represented the typical form of the Navicula brasiliensis of Grunow, 1 

 but the valves cannot be said to be "oval," nor the apices "very acute," as that author, 

 whose figure agrees entirely with the present one, has declared. In Grunow's frustule, 

 moreover, the central nodule is "large and subquadrate," but in the form now being 

 considered the raphe is interrupted by a subrotund areola. 



This form was obtained at the port of Tahiti. 



Plate XX. fig. 3, represents a very small navicular frustule from the same locality 

 in which the central areola is round. It accordingly corresponds more exactly with the 

 frustule described by Grunow. 



Navicula mammalis, n. sp. (Plate XX. fig. 2.) 



Elliptico-lanceolata ; apicibus mammiformibus ; lineis granulatis radiantibus, ad cen- 

 trum cessantibus. Ad mare Philippinarum. 



This form was obtained from, a sounding made in the neighbourhood of the Philippine 

 Islands. Its shape is elliptico-lanceolate, and it possesses mammiform apices. The 

 radiating granular strise extend from the periphery towards the median raphe, which, 

 however, they do not reach. The central nodule is surrounded by a smooth areola, which 

 is marked only by two arched shadows. 



The name of this species is derived from the character of its extremities. 



Navicula subrhomboidea, n. sp. (Plate XX. fig. 4.) 



Parva, lanceolato-rhomboidea ; apicibus cuneatis ; striis punctulatis, transversis, lineam 

 mediam attino-entibus. In mari Pacifico. 



1 Verhandl. d. k. li. zool.-bot. Gesellsch. in Wien % 1863, T. xiv. fig. 10, 



