REPORT ON THE DIATOMACE.E. 153 



arafuscensis, O'Me., the specific name being probably intended for arafurensis — since the 

 sea which extends from the Aru Islands to Torres Strait, where the specimen was found, 

 is called the Arafura Sea. The frustule is a little smaller than the preceding, from 

 which it may also be distinguished by the following circumstances, namely (1.) the sub- 

 hexagonal areola) decrease from the margin towards the centre and are stronger than in 

 Coscinodiscus craspedodiscus; and (2.) the central areola is smaller, loss stellate, and 

 irregular. 



The third form which was brought from Kerguelen is also an enormous disc. It has 

 been named Coscinodiscus moseleyi, O'Me. 1 It is very convex, and its central rosette 

 is formed by eight large unequal cellules or areolae and minute subquadrate granules 

 arranged in small radiating groups. 



In examining the numerous discoidal frustules which have now to be recorded, it is by 

 no means easy to determine the limits that are to be set to the genus Coscinodiscus. 

 Some of the granular or cellular discs correspond perfectly to the generic definition above 

 quoted, but several other forms possess so extremely delicate punctations as to surpass in 

 this respect all known species of Coscinodiscus, and to render it highly improbable that 

 they could have been observed by Ehrenberg with his comparatively imperfect microscope 

 when he established that genus. This view is confirmed by W. Smith, who, to prove that 

 his Coscinodiscus concinnus could not be confounded with the Coscinodiscus centralis of 

 Ehrenberg, remarks 2 that the cellules of the former could not have been detected by means of 

 the instrument used by Ehrenberg. But the punctations of the forms now in question 

 are even more minute than those of Coscinodiscus concinnus, so that they cannot be 

 regarded as conforming to the definition of that genus, but must be looked upon as 

 belonging to a new genus which I shall name Ethmodiscus 3 on account of the ex- 

 ceedingly fine condition of the granulation. 



Coscinodiscus arafurensis, O'Me., var. nov. (Plate II. fig. 4.) 



The frustule here figured is one of the largest members of the genus Coscinodiscus 

 that has to be recorded here. Its diameter is 349 M-, and it is ornamented with large 

 radiating cellules which become smaller towards the centre, where a smooth irregular 

 areola occurs. 



Though possessing the large size just noted the present valve is smaller than that of 

 Coscinodiscus arafurensis, O'Me., and much less than that of Coscinodiscus craspedo- 

 discus, O'Me. With the last-named frustule, however, it has been found to be associated 

 in a collection made in the Arafura Sea, although Coscinodiscus craspedodiscus, O'Me., 

 was also obtained in great numbers in a sounding of great depth taken in the Pacific Ocean 



1 Journ. Linn. Soc. Lond. (Botany), vol. xv. No. 82, p. 57, pi. i. fig. 6. 



2 Synopsis of the British Diatomaceae, vol. ii. p. 85. 



3 rfin,ti, a filter for liquids. 



(bot. chall. exp. — part iv. — 1886.) D 20 



