REPORT ON TI1E DIATOMACEdE. 145 



Actinocyclus complanatus, n. sp. (Plate IV. fig. 9.) 



Valvse ccllulis fasciculatim medio, propc niarginem decussatim ordinatis, hinc in 

 puuctulorutn lineis trauseuntes ; ad marginem rarse dentieulorum lincae distribuuntur ; 

 pseudonodulum intrarnargiuale evidens. In niari Japonico. 



This Diatom, which was collected in the Sea of Japan, corresponds in most of its 

 characters to Actinocyclus fasciculatus. Its form is discoidal, and the cellules, which 

 are equally distributed over the entire surface of the valve, are disposed in a fasciculate 

 manner. Its centre, however, is not specially differentiated, and the lines which radiated 

 from that region in Actinocyclus fasciculatus are here absent. In this respect, therefore, 

 the frustule does not correspond to the definition of the genus as given by Ehrcnberg, 1 

 but the presence of a well-defined submarginal pseudo-nodule clearly indicates its true 

 generic determination. 



The specific name that has been given has reference to the entire absence of salient or 

 other differential characteristics at the centre of the valve. 



Actinocyclus umbonatus, n. sp. (Plate IV. fig. 4.) 



Valvse umbonatas, cellulosse ; cellulse rarius in centro, reliquum fasciculatim ordinatae ; 

 zona marginalis dense lineata ; pseudonodulum submarginale cylindricum lateraliter 

 extensum. In mari Antarctico. 



This beautiful discoid umbonate Diatom was procured in the Antarctic Ocean. It 

 presents alternate radiating shades as in Podosira maculata, W. Sin., 2 these being due to 

 the distribution of the cellules in fasciculate parallel lines. Near the margin of the oblique 

 surface of the Diatom a small subulate cylindrical structure projects. This was observed 

 in many cases ; it always presented the same appearance, and from its oblique direction 

 it can only be the homologue of the pseudo-nodule, its position in the strongly umbonate 

 valve preventing it from appearing round as in other species of the genus. It is impor- 

 tant to note that the pseudo-nodule may assume this remarkable appearance, constituting 

 a well-marked process-like elevation on the surface of the valve. 



Actinocyclus oliveranus, O'Me. (Plate IV. fig. 7.) 



We have here represented one of the commonest and most characteristic forms that 

 occur in the Antarctic Ocean. It consists of a very small irregularly punctated disc 

 which possesses a pseudo-nodule near its margin. I had named the organism Actino- 

 cyclus antarcticus, but on comparing Part iv. of the Diatoms, edited by T. P. 

 Cleve and T. D. Moller, the same frustule was repeatedly observed by me in preparation 

 207 made from a sounding taken between Patagonia and the Sandwich Islands, and it 

 has already been designated Actinocyclus oliveranus, by the well-known Irish microscopist, 



1 Pritchard, op. tit., p. 833. 2 Smith, Synop. Brit. Diat., vol. ii. p. 54, pL xlix. fig. 328. 



(bot. chall. exp. — part iv. — 1886.) D 19 



