REPORT ON THE DIATOM ACEjE. 127 



This superb Diatom, from the South Atlantic, has a diameter of 186 m. Although at 

 first sight it seems to be identical with Eupodiscus argus, Ehrenb., 1 1 found, on comparing 

 it with the type of that species shown in the Typenplattcn of Moller, and with other 

 specimens in my possession, that it could not be confounded with the latter. Thus the 

 figure here given — which is correct in every way except that the punctation of the margin 

 is somewhat larger and thicker in the original, a character which could not be exactly 

 drawn without confusing the areolae — shows that these areolae are never stellate, but 

 irregularly circular. The three submarginal processes divide the perimeter of the valve 

 irregularly. The double structure, consisting of areolae and granules, observable in the 

 valve, as well as in that of Eupodiscus argus — a peculiarity which is also met with in the 

 species of other genera — seems to point to the existence in the cell walls of two strata, of 

 which one may sometimes become detached from the other, as if the more finely granulated 

 layer served as a lining for the other. 



Actinoptychus, Ehrenb. 



This genus was instituted by Ehrenberg in 1838, and was defined as follows: 2 — 

 "Frustules disciform, cellulose, disc divided into equal triangular compartments by lines 

 or internal septa." 



The term septa here employed is unfortunate, as it is used for those lines of division 

 which, in an almost diaphragm-like manner, cross the lumen of the diatomaceous cell. 

 e.g., in Striatella, Rhabdonema, &c. ; but in the present genus the lines of division 

 are but indications of the alternate depression and elevation of the compartments, and 

 are not septa in the usual sense. 



Although the general aspect of the genus Actinoptychus is similar to that of Omphalo- 

 pelta (Ehrenb.), Cstr., the latter is readily distinguished by the possession, in each of the 

 triangular compartments, of an intramarginal denticule or spine. 



Smith, in his Synopsis of the British Diatomaceae, has, on the other hand, confounded 

 the genus Actinoptychus with that of Actinocyclus — an error which resulted from his 

 having regarded as an Actinocyclus a species which was wrongly classed in this genus by 

 Kiitzing, instead of- being grouped with the species of Actinoptychus. 



Actinoptychus raeanus, n. sp. (Plate VII. fig. 4.) 



Forma rotunda, granulata, cellulosa; sex dissepimentis triangularibus distincta, cellulis 

 (areolis) subquadratis, denticuli grandiusculi plures intramarginales irregulariter distributi ; 

 umbilicus definitus hyalinus. Fossilis in deposito S te Monicas in California. 



Among the excellent preparations of Diatoms made from the Challenger collections by 



1 Smith, Synop. Brit. Diat, vol. i. p. 24, pL iv. fig. 39 ; Rabenhorst, Flora Europaea Algaram, sect. i. p. 

 319, &c. 



s Pritchard, op. cit., p. 839. Various species of this genus are figured in Ehrenberg's Mikrogeologie. 



