REPORT ON THE DIATOMACE/E. 99 



This Antarctic Diatom differs from the Mollcria cornuta of Cleve in the following 

 respects : — (1.) The greater size of the frnstules ; (2.) the greater shortness of the terminal 

 processes; (3.) the inferior degree of development of the connecting zone; and (4.) 

 the bipartite condition of the connecting zone ; the costa?, moreover, of the two hoops of 

 adjoining valves often diverge more or less — a peculiarity which has not been observed 

 in Cleve's species. Although taken singly, these differences probably are not of very 

 great importance ; it cannot be doubted that the combination of all of them is sufficient 

 to warrant the establishment of this new species. 



Hemiaulus, Ehrenb. 



Seeing that the frustules of this genus are provided with two processes terminated by 

 a thorn or claw, we are led to conjecture, in opposition to the assertion made by Pritchard, 1 

 that the frustules exist in a concatenate or seriate form, since the spine can only be inter- 

 preted as a connecting organ. This conjecture I have been able to verify more than once 

 by observing two or more frustules linked together, and in such a manner that their ter- 

 minal spines constantly alternate with one another. Thus of the two spines of one valve, 

 one is superposed to the corresponding spine of the adjoining frustule, while the other is 

 covered by its corresponding spine. This peculiar method of union may also be studied 

 in the genus Ch&toceros, but its signification is by no means easy to understand. The 

 concatenation of Hemiaulus was observed, and accurately described, by Heiberg, in his 

 work entitled Conspectus criticus Diatomacearum Danicarum, plate i. a. 



In Hemiaulus, as in Cliietoceros and Cocconeis, the terminal valve differs somewhat 

 from the others — a circumstance which must be carefully borne in mind in order not 

 to multiply species unduly. The overlooking of this fact explains why, in Pritchard's 

 History of the Infusoria, the different condition of the processes, as elongated in one valve 

 and truncated in the other, is given as a specific character of Hemiaulus antarcticus, 2 

 Ehrenb. But the processes of the terminal valve are truncated in all the species, since 

 the spine becomes superfluous where union with another frustule does not take place. 



Hemiaulus, sp. ? (Plate XXI. figs. 9 and 13.) 



The two valves of Hemiaulus here figured must be regarded as terminal, but their 

 specific determination must for the present remain doubtful. The only circumstance — ■ 

 apart from the more crowded condition of the granules — which would seem to indicate 

 that the valve represented in fig. 13 belongs to a distinct species from that shown in 

 fig. 9, is the existence of a well-defined granule, surmounting its two obtuse terminal 

 processes, instead of the simple spine or process which is peculiar to the latter. 



1 Pritchard, op. cit., p. 851. 



2 Ehrenberg, Mikrogeologie, pi. xxxv. A 22, fig. 15 ; Pritchard, op. cit., pi xi. fig. 54. 



