EEPOET ON THE DIATOMACE/E. 69 



Their apices were very acute, and the valves finely striated, while they were at the same 

 time surrounded by a row of small granules or lines similar to those which occur in the 

 NitzschioB. 



Bacillaria socialis, Greg., var. indica, nov. (Plate XXV. figs. 9 and 10.) 



The known species of Bacillaria are few, and amongst these Pritchard (op. cit., p. 784) 

 records as Bacillaria socialis a form determined by Gregory 1 as Nitzschia socialis. On 

 comparing this frustule with those from the Arafura Sea they are found to agree (1.) in 

 having their valves of a linear-lanceolate form ; (2.) in being ornamented with a very fine 

 transverse striation ; and (3.) in presenting acute apices ; while they differ in the following 

 respects : (1.) The Arafuran frustules are slightly sigmoid, while the others do not exhibit 

 this peculiarity ; (2.) the presence of a central keel, mentioned by Gregory as occurring 

 in his Diatom, is not to be detected in the present case, in which there is (3.) a band of 

 granules which are somewhat more distinct and more sparsely distributed than the striae. 



Although, in the absence of an authentic preparation of Gregory's species for purposes 

 of more accurate comparison, it is not easy to determine the true significance of the above- 

 mentioned points of difference between what are otherwise two closely allied forms, the 

 frustules now figured have in the meantime been regarded as a variety of his typical form. 



TRIBE III — CRYPTORAPHIDIK& 



Rhizosolenia, Ehrenb. 



Judging from the various definitions that have been given of this genus, which was 

 first introduced by Ehrenberg, 2 it does not appear to have been clearly understood hitherto 

 by any observer. The definition given by Pritchard in his History of the Infusoria (p. 865) is 

 as follows : — " Filamentous, frustules subcylindrical, greatly elongated, siliceous, annulate ; 

 annuli broadly cuneate ; surface striated, extremities calyptriform, pointed with a bristle." 

 In the Micrographic Dictionary, on the other hand, the following generic characteristics 

 are pointed out : — " Frustules elongate, subcylindrical, marked with transverse spiral lines, 

 ends oblique or conical, and with one or more terminal bristles." But these definitions 

 cannot be said to correspond more accurately to the reality than that proposed by 

 Ehrenberg himself, which, according to Pritchard, was as follows : — " Lorica tubular, with 

 one extremity round and closed, while the other is attenuate and multifid as if terminating 

 in little roots." 



It is not easy to understand why it was not recognised from the beginning that the 



1 Trans. Micr. Soc. Lond., vol. v. p. 80, pi. i. fig. 45. 



2 Ehrenberg, Hikrogeol., plates xviii., xxxiii., and xxxv. 



