REPORT ON THE DIATOMACEiE. 37 



The zonal side of the frustules is so much narrower than the valval as to render it 

 almost impossihle to meet with an individual that shows anything more than the plane 

 of the valve. This is generally elevated, and, when seen with the binocular stereoscopic 

 microscope of Nachet, shows itself in the form of an extremely depressed pyramid with a 

 very extended basis, so as to cause some doubt in the case of species in which the raphe 

 divides the extremities of the valve unequally, as in Pleurosigma formosum, W. Sin., 

 Pleurosigma decorum, W. Sm., Pleurosigma balticum, W. Sm., &c, whether this 

 appearance may not be the effect of a difference in obliquity at that point of the extreme 

 sides of the pyramid. 



Pleurosigma elegantissimum, n. sp. (Plate XXVIII. fig. 1.) 



Valvis gracillime lanceolatis, rotundato-acutis ; linea centrali excentrica medio iuflexa ; 

 striis obliquis delicatissimis. Ad mare Japonicum. 



This frustule was found in a sounding made near Yedo in the Sea of Japan. The 

 striae, which are very delicate, are arranged in a decussate manner. The form of the 

 valve is slightly lanceolate, and the extremities are acute. The profile reminds the observer 

 of the Pleurosigma delicatulum of W. Smith, 1 but it may be at once distinguished from 

 the latter by the fact that the raphe divides unequally not only the extremities but also 

 the general surface of the valve. Moreover, towards the central nodule the raphe of 

 Pleurosigma elegantissimum is depressed so that the sigmoid line appears broken. 



From such considerations the specific value of this form cannot be doubted. 



Pleurosigma naviculaceum, Breb., var. nov. (Plate XXVIII. fig. 3.) 



AVe have here represented a very minute Pleurosigma which was found in the 

 Arafura Sea, where many other very interesting forms have been obtained. It is remark- 

 able on account of its lanceolate profile and its greatly sigmoid raphe, which divides the 

 two halves of the valve unequally. The central nodule is oval and very large, and in 

 the closest proximity to this on both sides there is a well-defined smooth areola. The 

 extremities are somewhat lengthened and rounded, and slightly recurved in a sigmoid 

 manner. 



Similar characteristics are found in Pleurosigma naviculaceum, 2 Breb., with the 

 exception of the double indistinct areola near the central nodule. 



Pleurosigma speciosum, W. Sm., var. nov. (Plate XXVIII. fig. 2.) 



This figure shows another linear Pleurosigma, which was obtained in a sounding 

 taken at the port of Tahiti. The striae, which are arranged in a decussate manner, are 



1 W. Smith, op. cit., vol. i. p. 64, pi. xxi. fig. 202. 



2 Br^bisson, Diat. de Cherbourg, p. 17, fig. 7 j = (1.) Gyrosigma transversale, Microg. Dictionary, 1854, 

 pi. xl figs. 37 and 38; (2.) Pleurosigma transversale, W. Smith, op. cit., vol. ii. p. 96. 



