264 DISCOVERY REPORTS 



Family HEMIDISCACEAE 



Subfamily HEMIDISCOIDEAE 



Cells solitary, valve oval-semicircular to sublunate, cuneate in girdle view, surface minutely 

 punctate, radiate, eccentric, numerous marginal spinulae, small ocellus upon ventral 

 margin ... ... ... ... ... ... ... ... •■■ ••• Hemidiscus 



Genus Hemidiscus Wallich 

 Wallich, i860 



After examining a great number of specimens, Castracane (1886) came to the con- 

 clusion that no real difference existed between the genus Euodia Bailey and Hemidiscus 

 Wallich, and desired to unite them under one generic name. He stated that: " Since, 

 then, the genus Euodia, Bail, was instituted prior to that of Hemidiscus, Wall, the name 

 of the united genera must be Euodia, Bail." Castracane is in error here, for the reverse 

 is the case. The genus Euodia was established in 1861 and Hemidiscus in i860. 



Hemidiscus cuneiformis Wallich. 



Wallich, i860, p. 42, pi. 2, figs. 3-4. 



Hustedt, 1930, p. 904, fig. 543 e. 



Euodia cuneiformis Schiitt, 1896, p. 100. 



Euodia radiata Castracane, 1886, p. 150, pi. 12, fig. 4. 



Euodia inornata Castracane, 1886, p. 149, pi. 12, fig. 1. 



Cells cuneiform, solitary. Valves almost semicircular; dorsal margin strongly convex, 

 ventral margin weakly so, often furnished with a median inflation upon the ventral side. 

 Apices rounded, sometimes slightly produced. Valve covered with fine areolation 

 arranged in irregularly radial fascicules; areoles in short parallel lines particularly to- 

 wards the centre of the valve, irregular or in tangential sectors towards the margin and 

 apices. Central hyaline area and rosette absent. Apices furnished with a small incon- 

 spicuous process. Upon the ventral margin of the valve is a row of small spinulae, and 

 a small pseudo-ocellus occupying a position half-way between the apices. Girdle wide 

 upon the dorsal side and narrow upon the ventral side of the frustule, giving the cell the 

 appearance of a quarter of an orange. Girdle simple, no intercalary bands. Chromato- 

 phores: numerous small rounded bodies. Diameter of valve, apical axis 80-1 74^; 

 transapical axis 44-90/u,. 



Castracane (1886) referred species of this genus to Euodia Bailey (v.s.). Hemidiscus 

 cuneiformis has a wide distribution throughout tropical and subtropical seas, and has 

 been observed occasionally in north European waters. It is an oceanic species. 



Much variation in outline has been observed, and many species and varieties have 

 been created upon such variable characters as the ratio between the apical and trans- 

 apical axes, the degree of gibbosity displayed upon the ventral side, the convexity of the 

 dorsal side, and the development of the marginal spinulae and pseudo-ocellus. From 

 the vast amount of material examined from such widespread areas as the waters around 



