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



constriction three times as long as the lentelliptical medullary shell. Four cupolas obliquely 

 elliptical. 



Dimensions. Length of the cortical shell O16, breadth 0'12 ; length of the medullary shell 0'04, 

 breadth 0'03. 



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



Genus 302. Zoniscus, 1 n. gen. 



Definition. Z o n a r i d a with six dome-shaped chambers of the cortical shell, 

 separated by three annular constrictions (one sagittal and two transverse, parallel 

 to the equatorial plane). 



The genus Zoniscus differs from Zonarium as well as from Zonidium by the 

 development of the transverse girdle, which is not constricted, but on the contrary 

 prominently vaulted in the equatorial plane. Therefore both wings of the transverse 

 girdle form here two opposite lateral or " equatorial cupolas," as in Amphitholus. 

 These are separated from four other domes (the " corner cupolas ") by two transverse 

 annular constrictions, which correspond to the free edges of the original transverse 

 girdle. The corner domes of each pair are separated from each other by the 

 sagittal septum. 



1. Zoniscus rectangulus, n. sp. 



Cortical shell nearly rectangular, with rounded corners, nearly one and a half times as long as 

 broad. Surface thorny, without larger radial spines. Sagittal constriction scarcely half as long as 

 the hexagonal medullary shell. Both equatorial cupolas (or wings of the transverse girdle) 

 scarcely half as large as the four corner cupolas. 



Dimensions. Length of the cortical shell 0'15, breadth Oil ; length of the medullary shell 

 0-07, breadth 0'045. 



Habitat. North Pacific, Station 253, depth 3125 fathoms. 



2. Zoniscus hexathalamus, n. sp. 



Cortical shell nearly elliptical, one and a third times as long as broad. Surface nearly smooth, 

 without radial spines. Sagittal constriction three times as long as the lentelliptical medullary 

 shell. Both equatorial cupolas about as large as the four corner cupolas. 



Dimensions. Length of the cortical shell 0'12, breadth 0'09 ; length of the medullary shell 

 0-035, breadth 0'025. 



Habitat. North Pacific, Station 244, depth 2900 fathoms.. 



1 Zoniscus = Elegant girdle ; asu'azo;. 



