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



not very minute. The central line or raphe is throughout the greater part of its course 

 straight but curves near the extremities of the valve, so that the surface of the latter 

 is divided into two equal parts while the raphe runs obliquely across it. 



The Pleurosigma speciosum of W. Smith 1 agrees with the present form in every 

 respect except with regard to the profile of the valve, which, instead of being linear, is in 

 the former linear-lanceolate. Hence the Tahitian frustule has been regarded as a variety 

 of Smith's species. 



Pleurosigma arafurense, n. sp. (Plate XXVIII. fig. 5.) 



E minimis, lanceolata, vix sigmoidea ; apicibus obtusis ; linea media excentrica ; nodulo 

 centrali grandiusculo, quadrato ; striis decussatis tenuissimis. In mari Arafura. 



This very minute species, from the Arafura Sea, is provided with excessively delicate 

 strioe. It. is remarkable on account of the size and subquadrate form of the central nodule 

 as well as the very slight sigmoid curve of the profile of the valve. The median raphe is 

 central and the extremities are obtusely rounded. 



The specific name has reference to the locality from which it has first been recorded. 



Pleurosigma thaitiense, n. sp. (Plate XXVIII. fig. 4.) 



Valvis lanceolatis, gradatim ad apices acuto-rotundatos terminantibus, et moderate 

 curvatis ; raphe centrali ad apices submarginali ; striis decussatis vix perspicuis. In 

 portu Thaiti. 



This very elegant frustule possesses a lanceolate sigmoidal form and acutely rounded 

 extremities. The flexion is. moderate, and the central raphe becomes decidedly excentric 

 towards the ends. The granulation is very delicate and decussate. 



Pleurosigma smithianum, n. sp. (Plate XXVIII. fig. 6.) 



Valvis lanceolatis, acutis, aduncis ; raphe sigmoideo, ad centrum oblique transverso, 

 dein submarginali. Ad portum Thaiti. 



In the collection which contained Pleurosigma thaitiense, n. sp., from the port of 

 Tahiti, the present frustule was often observed. The disposition of the raphe is particu- 

 larly noteworthy, remaining submarginal at the two extremities, then closely following 

 the outline of the valve, and finally abruptly crossing it near the centre in an elegant curve. 

 The extremities of the valve are acute and somewhat hooked. This form is extremely 

 characteristic and very easy to determine. 



Pleurosigma japonicum, n. sp. (Plate XXIX. fig. 14.) 



Minimum, sigmoideo-lanceolatum ; linea media apices cuneato-obtusos imequaliter 

 dividente ; grandiusculorum punctulorum lineis decussatis. Ad mare Japonicum. 



1 W. Smith, op. tit, vol. i. p. 03, pi. xx. fig. 197. 



