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



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



Definition. Ty m pan id a with two simple horizontal rings, connected by two 

 vertical columellae. 



The genus Parastephanus is the simplest among the Eutympanida, and exhibits 

 only two vertical rods or columellse, which connect the two parallel horizontal rino-s. 

 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. Parastephanus circularis, n. sp. 



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

 which are somewhat shorter than the diameter of the rin^s 



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Dimensions. Diameter of the rings 0'08, length of the columellae 0'06. 

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



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



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

 vertical, slightly curved columellse, 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 columellae G"05. 



Habitat. North Pacific, Station 236, surface. 



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



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

 what asymmetrical form. The two connecting parallel columellfe are straight, cylindrical, about as 

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

 possibility that the two columellaa 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 columellas 0'08. 



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



