REPORT ON THE RADIOLARTA. 1009 



4. Parastephanus dispar, n. sp. 



Two horizontal rings elliptical, thorny, of different sizes, the basal ring one and a half times as 

 broad as the mitral ring. The two connecting columellse are curved, somewhat longer than the 

 diameter of the mitral ring. 



Dimensions. Diameter of the basal ring O09, mitral 0'06 ; length of the columelke 0'07. 



Habitat. South Pacific, Station 300, depth 1375 fathoms. 



Genus 436. Prismatium, 1 Haeekel, 1862, Monogr. d. Radiol., p. 270. 



Definition. T ympanida with two simple horizontal rings, connected by three 

 vertical columellse. 



The genus Prismatium was hitherto the only known form of the Tympanida, and the 

 only species described in 1862 in my Monograph was Prismatium tripleurum (foe. cit., 

 p. 270, pi. iv. fig. 6). A new species from the Challenger collection (PL 93, fig. 22), 

 Prismatium tripodium, explains the true structure of this remarkable genus. The skeleton 

 is composed of nine siliceous rods, corresponding to the edges of a trilateral prism. 

 Six of these are horizontal, and enclose the two parallel triangular horizontal gates, 

 the superior of which is formed by the mitral or coryphal, and the inferior by the basal 

 or cortinar ring. The three other rods are vertical and nearly parallel, and connect (as 

 lateral edges of the prism) the corresponding corners of the two parallel horizontal 

 triangles. Two of these vertical columellse, together with the two horizontal parallel rods 

 connecting them, represent . the complete frontal ring, whilst the third vertical columella 

 is the posterior half of the sagittal ring, the other parts of which are lost. 



1. Prismatium tripodium, n. sp. (PI. 93, fig. 22). 



Mitral ring triangular, somewhat smaller than the triangular basal ring, both connected at the 

 corners by three vertical, slightly curved columelLs. From the six corners arise six radial spines 

 of different size and form ; the two paired spines of the mitral ring are simple and very small, 

 whilst those of the basal ring are three times as long and bear a lateral branch. The odd spine of 

 the third corner is on both rings larger, and distinguished by some thorns. This difference seems 

 to indicate that these two odd spines correspond to the apical horn and the caudal foot of Cortina, 

 and the eolumella between them is the rest of the reduced sagittal ring. In this case the quad- 

 rangular vertical ring, which is composed of the two paired columellse and the two connecting 

 horizontal rods, would be probably the frontal ring. 



Dimensions. Length of the shell 0'09, breadth 0'07, 



Habitat. Equatorial Atlantic, Station 347, surface. 



1 Prismatium=A small prism ; x-eiaftanei. 

 (ZOOL. CHALL. EXP. PAST XL. 1886.) Er 127 



