REPORT ON THE RADIOLARIA. 983 



The genus Tristephanium, and the three following genera derived from it, represent 

 together the interesting subfamily of Trissocyclida (Prodromus, 1881, p. 446). These 

 differ from all other Coronida in the possession of three complete rings, perpendicular 

 to one another. The first of these is the vertical sagittal ring, the second the vertical 

 frontal ring, and the third the horizontal basal ring. Since these three rings lie in the 

 three dimensive planes, they are perpendicular to one another ; and between them 

 remain eight large open gates. Originally the four upper or lateral gates (corresponding 

 to those of Zygostephanus) are much larger, the four lower or basal gates (corresponding 

 to those of Semantrum) much smaller ; but afterwards the latter may reach the size of 

 the former, so that the basal ring becomes equatorial (in Trissocircus and Trissocyclus). 

 The common ancestral form of the Trissocyclida (Tristephanium) may be derived directly 

 either from Semantrum (by complete development of the frontal ring) or from 

 Eucoronis (by complete development of four basal gates). 



Subgenus 1. Triostephus, Haeckel, 1881, Prodromus, p. 445. 

 Definition. Sagittal and frontal ring of different size and form. 



1. Tristephanium dimensivum, n. sp. (PI. 93, fig. 9). 



Sagittal ring ovate, with a large thorny apical horn, smaller than the two other rings. Frontal 

 ring violin-shaped, with a deep sagittal constriction. Basal ring also violin-shaped, in the sagittal 

 axis constricted. Its four gates (two jugular and two cardinal) of nearly equal size. All three 

 rings angular, armed with scattered, stout, thorny spines, which are larger in the basal half. 



Dimensions. Height of the frontal ring 0'08, breadth - 12. 



Habitat. North Pacific, Station 241, depth 2300 fathoms. 



2. Tristephanium octopyle, n. sp. (PI. 93, fig. 8). 



Sagittal ring ovate, nearly smooth, smaller than the two other rings. Frontal ring kidney- 

 shaped, with a deep basal constriction. Basal ring violin-shaped. The jugular gates about half as 

 large as the cardinal gates. Eods of the rings cylindrical, with few scattered thorns. 



Dimensions. Height of the frontal ring 0'09, breadth 0'14. 



Habitat. Central Pacific, Station 271, depth 2425 fathoms. 



3. Tristephanium hertwigii, Haeckel. 



Acanthodesmia hertwigii, Biitschli, 1882, Zeitschr. f. wiss. Zool., vol. xxxvi. pp. 499, 539, 

 Taf. xxxii. figs. 9a-9e. 



Sagittal ring ovate or nearly semicircular ; its dorsal rod straight, smooth (fig. 9c, a), its ventral 

 rod (&) curved, thorny. Frontal ring much larger, thorny, violin-shaped, with a slight sagittal con- 



