262 DISCOVERY REPORTS 



Actinocyclus Janus Karsten. 



Karsten, 1905, p. 92, pi. 9, fig. 7. 



Cells discoid, solitary. Valves dissimilar. Upper valve slightly convex, usually 

 flattened at the centre. Valve surface covered with puncta. Puncta arranged in irregular 

 concentric lines, sparse towards the centre of the valve, often leaving a small central 

 hyaline space. Pseudo-ocellus small, marginal, oval. The lower valve is more deeply 

 convex, particularly towards the margin, very much flattened at the centre, sometimes 

 slightly depressed. Valve surrounded with a broad margin, radially striate, striae strong. 

 Pseudo-ocellus oval to globiform, situated upon the striate margin. Valvar portion 

 covered with puncta, sparse at the centre, and irregularly arranged. Puncta more dense 

 towards the margin and in irregular concentric lines. Chromatophores : numerous small 

 bodies. Diameter of valve 52-58/x. 



Observed at Sts. 378, 379, 380, 382, 383, 475, 477, 478, 479, 480, 501, 503, 505, 506, 

 508. 



Actinocyclus octonarius Ehrenberg. 



Ehrenberg, 1838, p. 172, pi. 21, fig. 7. 



Actmocyclus Ehrenbergii Ralfs ex Pritchard, 1861, p. 834. 



Cells discoid, solitary. Valves convex, deeply so at the margin, central area often 

 flattened. Valve covered with moniliform striae. The striations consist of a varying 

 number of radial lines which proceed from the margin of the valve to a small cluster of 

 puncta at the centre, dividing the valve surface into radial compartments. Each com- 

 partment is furnished with a fascicule of lines of puncta, the lines being parallel to the 

 median or radial line of each fascicule. As the fascicule does not completely fill the com- 

 partment that contains it, the main radial lines forming the compartments appear to be 

 surrounded by hyaline interfascicular spaces, so that when the valve is viewed with a low 

 power objective, it has the appearance of being divided by a number of transparent 

 radial rays. Peripheral area more finely striate, and furnished upon the outer zone with 

 marginal apiculi. A narrow outer margin is finely radially striate. When viewed with a 

 low power objective (i-J in.) the valve has the appearance of possessing concentric 

 zones of beautiful colours, but these are diffraction effects which disappear when an 

 objective of high numerical aperture is used. Marginal pseudo-ocellus present, small, 

 rounded. Girdle plain, simple, no intercalary bands. Chromatophores : numerous small 

 plates. Diameter of valve 1 70-220/x. 



A neritic species, having a cosmopolitan distribution ; particularly numerous in tem- 

 perate seas. 



It has been explained under the title of the genus, how this species became the type. 

 Two species were described by Ehrenberg when the genus was created, but one of 

 them, namely Actinocyclus senarius, was removed to become the type of the genus 

 Actinoptychus (Ehrenberg, 1843, p. 400). The number of compartments present upon 

 the valve surface may vary very considerably. Ehrenberg found that it varied from 



