80 THE VOYAGE OF H.M.S. CHALLENGER. 



The small but distinct projecting processes which are sometimes present in the centre 

 of the valves have also been regarded as having but a varietal significance. 



The specific name which has been adopted, will serve to remind the observer of the 

 curious form of the setse or processes. 



Chaetoceros, sp. (?) nov. 



The annexed woodcut represents a Chsetoceran valve with long denticulate awns, 



but hitherto insufficient material has been observed 

 to enable a complete account to be given of its 

 specific characters. It must, therefore, be indicated 

 as a doubtful species. It was obtained in a surface 

 collection made to the south of Heard Island. 



Chaetoceros incurvum, Bail., var. umbonatum, nov. 

 (Plate XXIX. figs. 10 and 16.) 



Chaetoceros, sp. (?) nov, _ 



An examination of the contents of the alimentary 

 tube of two Echini dredged in the North Atlantic at Station 47, from a depth of 1340 

 fathoms, revealed the presence of numerous Diatoms, which were no doubt serving as 

 food to the animals in question. One of the most frequent of these was the very small 

 Chaetoceros shown in the present figures. Its oval valves are provided with short re- 

 curved filaments, and the frustules are never found in series. Hence the entire genus 

 Clisetoceros may be divided into two sections, namely — (a.) Clisetocerotidse gregarise, com- 

 prising all those forms that occur in chains, and (b.) Chsetocerotidse solitaries, embracing 

 all free species — as, e.g., Clisetoceros radiculum, and the form now before us. 



The chief point of distinction between this form and Chaetoceros incurvum, Bail., 1 

 consists in the notable swelling at the centre of the valve which is to be found in the 

 former. This, however, can hardly be regarded as a difference of specific importance. 



In the genus Chaetoceros, I include three genera established by Ehrenberg, 2 namely — 

 Dicladia, Goniothecium, and Syndendrium. These have from the first presented great 

 difficulties, and, in the words of Brisrhtwell, 3 " much must vet be brought to light before 

 a satisfactory classification of this group can be effected." The uncertainty in connection 

 with them is owing to the fact that they have hitherto only been observed in a fossil 

 condition in deposits, so that it has been found impossible to understand their form 

 when in a state of actual vegetation sufficiently well to enable the observer to ascribe to 

 them their proper place in a system of classification. 



1 Micr. Journ., voL iv. pL vii. figs. 9-11. • Ehrenberg, Mikrogeologie. 



3 Quart. Jirum. Micr. Set., vol. iv. p. 105-109, pi. vii. 



