438 THE VOYAGE OF H.M.S. CHALLENGEE. 



Genus 191. Heliosestrum,^ Haeckel, 1881, Prodroirras, p. 457. 



Definition. — P h a c o d i s c i d a with simple medullary shell and with eight radial 

 spines on the margin of the disk (more or less regularly disjjosed, sometimes seven or 

 nine). 



The genus Heliosestrum (with simple medullary shell) and the ioW^owmg Astrosestruin 

 (with doable medullary shell) contain those Phacodiscida in which the margin of 

 the lenticular disk bears eight radial spines, commonly more or less regularly disposed 

 in the equatorial plane, so that the equal angles between them amount to 45°. There 

 are, however, many exceptions to this regular eight-rayed form, either the angles between 

 the eight spines becoming unequal or the number of the spines amounting to seven or 

 ]iine (sometimes also sis or ten) in one and the same species. But the regidar form is 

 so prevalent, and also in the abnormal forms indicated by the position of the spines, 

 that we separate the genus Heliosestrwn from Heliodiscus. Both genera are rich in 

 common species, and in some of the former four perradial (larger) spines alternate 

 regularly with four interradial (smaller) spines. 



Subgenus 1. Heliosestantha, Haeckel. 



Definition. — Surface of the disk smooth, without radial spines. Bases of the 

 marginal spines free, not connected by an equatorial girdle. 



1. Heliosestrum medusiiturn, n. sp. (PL 34, fig. 6). 



Heliodiscus meduginus, Haeckel, 1881, Atlas (pi. xxxiv. iig. 6). 



Disk with smooth surface, four times as broad as the medullary shell. Tores regular, circular ; 

 about ten on the radius of the disk. Eight marginal spines regularly distributed (sometimes seven 

 or nine, more or less irregular), angular, nearly pyramidal (with equilateral triangular outline), about 

 as long and broad as the diameter of the medullary shell, without connecting equatorial girdle. 

 From the broad base of each spine run eight to ten deep furrows convergent to its ajjex. 



Dimensions. — Diameter of the disk 0-2, of the medullary shell 0-05 ; length of the radial spines 

 0-05, basal breadth 005. 



Habitat. — North Pacific, Station '2iMi, surface. 



2. Heliosestnim octastrum, n. sp. 



Disk with smooth surface, three times as broad as the medullary shell. Pores hregular, 

 roundish ; eight to nine on the radius. Eight marginal spines regularly distributed (sometimes 



