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



This very delicate form, from the port of Tahiti, possesses extremely delicate striae. 

 The valve is linear in form, and its extremities are cuneately rounded and sometimes 

 slightly protracted. The specific name has reference to the parallel disposition of the striae. 



Navicula, sp. (?) (Plate XXVIII. fig. 13.) 



We have here represented another naviculoid very finely fluted frustule from the same 

 collection, and remarkable from the fact that its raphe is bent. This appearance may have 

 resulted during the final stages of development, when growth in the centre of the valve 

 may have continued after that at the circumference had ceased, thereby producing the 

 undulating raphe referred to. 



Plate XXVIII. fig. 17, represents a curious lanceolate Navicula, provided with a 

 broad rounded median region which tapers towards the narrow obtuse extremities. The 

 central nodule is larcre and round, but the striae are almost invisible in Canada-balsam 

 preparations. Since all its characteristics could not be determined, its specific value must 

 at present remain undecided. 



In Plate XXVIII. fig. 15, a naviculoid frustule from the collection made at the port of 

 Tahiti is delineated in its zonal aspect. From this point of view it possesses a biconvex 

 outline, the raphe, however, is low at the centre of the valves. 



The magnificent frustule represented on Plate XXVIII. fig. 9, differs from Navicula 

 spectabilis, Grev., in the elliptico-rhomboidal form of its valve — a distinction which, 

 however, cannot be regarded as of sufficient importance to justify the establishment of a 

 new species for this form. 



Navicula decipiens, n. sp. (Plate XXVII. fig. 17.) 



Magna, late lanceolata, apicibns rotundatis ; striis transversis subtilissimis ; nodulo 

 centrali minimo. In mari Philippinarum. 



This frustule may at first sight be regarded as belonging to the genus Amphora, 

 since, like the latter, it presents in the middle region two small noduli. It is to be noted, 

 however, that the two noduli are not at the same level, while the two peripheral lines 

 belong to different valves which are closely united by a connecting zone. 



The form of the frustule and the very minute transverse striation may also lead the 

 observer to regard it as being identical with Navicula ostrearia, 1 Kg., but the size of the 

 different specimens that may be observed, and the extreme smallness of the central nodule, 

 oppose this view. Moreover, the proximity of the two valves argue its distinct specific 

 character. From the apparent agreement with the genus Amphora the specific name 

 decipiens has been chosen for this form. 



1 Kiitzing, Species Algarum, p. 77. 



