1008 THE VOYAGE OF H.M.S. CHALLENGER. 



Genus 435. Parastephanus, Haeckel, 1881, Prodromus, p. 446. 



Definition. — T y m p a n i d a with two simple horizontal rings, connected by two 

 vertical columellse. 



The o-enus Parastephanus is the simplest among the Eutympanida, and exhibits 

 only two vertical rods or columellse, which connect the two parallel horizontal rings. 

 One of these latter must be regarded as the mitral or superior ring, the other as the 

 basal or inferior ring ; whilst the two parallel columellse, connecting both, are the 

 remaining vertical parts of the sagittal ring, the upper and lower parts of which are lost. 

 By loss of the latter Parastephanus has probably been derived from Protympanium. 



1. Parastepha7ius circularis, n. sp. 



Two horizontal rings smooth, circular, of equal size, connected by two curved smooth columelkc, 

 which are somewhat shorter than the diameter of the rings. 



Dimensions. — Diameter of the rings 008, length of the columellse 0'06. 

 Habitat. — Central Pacific, Station 263, depth 2650 fathoms. 



2. Parastephanus quadrispinus, n. sp. (PL 93, fig. 21). 



Two horizontal rings smooth, elliptical or nearly circular, of equal size, connected by two 

 vertical, slightly curved columellas, which are about as long as the radius of the rings. Each ring 

 bears two simple lateral spines, which in the mitral ring are curved downwards, in the basal ring 

 upwards, possibly the last remains of a reduced frontal ring. 



Dimensions. — Diameter of the rings 0-09, length of the columellse 0'05. 



Habitat. — North Pacific, Station- 236, surface. 



3. Parastephanus asymmetricus, n. sp. (PI. 82, fig. 10). 



Two horizontal rings thorny, of equal size, irregularly ovate or nearly semicircular, and of some- 

 what asymmetrical form. The two connecting parallel columellffi are straight, cylmdrical, about as 

 long as the diameter of the rings. The peculiar asymmetry in the form of the rings raises the 

 possibility that the two columelke in this species are the remaining parts of a frontal ring, and 

 that the sagittal ring has altogether disappeared. In this case the convex half of both rings would 

 be the ventral, the straight half the dorsal part. 



Dimensions. — Diameter of the rings 01, length of the columeUse 0'08. 



Habitat. — Central Pacific, Station 272, depth 2600 fathoms. 



