36 THE VOYAGE OF H.M.S. CHALLENGEK. 



that the present frustule differs from Cleve's species in the fact that the curvature of the 

 middle line is much less — a distinction which is probably due to the fact that we have to 

 deal with different stages of development in the two cases, and one which is not of great 

 importance. 



This form occurs pretty frequently in the sea around the Philippine Islands as well 

 as in the Sea of Japan. 



The form represented in Plate XV. fig. 5, is also closely related to the same species. It 

 possesses, however, a somewhat rhomboidal profile and the extremities are more obtuse, 

 the raphe being at the same time more excentric. Notwithstanding these differences, it 

 must be regarded as merely a varietal frustule. 



Plate XXVIII. fig. 14, represents another variety of Alloioneis antillarum, CI. et Grun., 

 which differs from the latter only in the much greater dimensions of the frustule and in 

 the proportion between the longitudinal and transverse axes. 



Alloioneis japonica, n. sp. (Plate XX. fig. 12, bis.) 



Linearis elongata ; apicibus cuneato-rotundatis ; striis laxis, transversis, ad apices 

 subradiantibus, hue lineam mediam attingentibus, illuc brevioribus et lsevem aream 

 linquentibus. Ad mare Japonicum. 



This curious navicular form was found in mud procured from a sounding of 335 

 fathoms in the Sea of Japan. It possesses along narrow elliptical form and terminates in 

 two cuneato-acute extremities. The striae, which on one side touch the raphe, do not reach 

 it on the other side, and near the central nodule they leave a well-marked smooth area. 



Pleurosigma, W. Smith. 



The genus Pleurosigma is perhaps the most remarkable among the navicular Diatoms. 

 The surfaces of the valves are adorned with very minute granules, and this fact, as is well 

 known, is often made use of in testing the quality of objectives for microscopes. 



The genus includes all those navicular forms which possess a granulated surface, and 

 whose valves, or at least central raphes, are curved in a sigmoid manner. The name 

 Gyrosigma was that first given by Hassall 1 to such forms, but the word Pleurosigma, 

 which was introduced by W. Smith, has now been universally adopted, being more in 

 conformity with the rules of nomenclature, and also because of the accuracy maintained 

 by that observer in his revision of the species which belong to this group. 



The valves of all species of the genus are covered with very minute equal granules 

 arranged in a quadrate or in a decussate quincuncial order, and upon these character- 

 istics Smith based his general system. 



1 Pritchard, op. cit., pp. 915, 916. 



