REPORT ON THE RADIOLARIA. 885 



The genus Diploconus must be derived from Hexacolpus (among the Hexalaspida) 

 by the stronger development of the two hydrotomical spines and their large sheaths ; 

 all the other eighteen spines are much smaller, and usually devoid of prominent sheaths. 

 Sometimes the eight tropical spines are rudimentary. 



Subgenus 1. Diploconulas, Haeckel. 



Definition. Mantle of the double cone not compressed ; its transverse section 

 therefore circular. 



1. Diploconus amalla, n. sp. (PL 140, fig. 1). 



Mantle of the double cone thick walled, not compressed ; its transverse section circular ; its 

 contour little convex ; its surface nearly smooth ; its distal margin regularly denticulated. 

 Diameter of its mouth twice as large as the equatorial diameter of the shell and one-third as long 

 as its total length. The two large spines prismatic, one-fourth longer than their conical sheath. 

 The eighteen smaller spines very thin, about half as long as the former, a little curved. 



Dimensions. Length of the shell O3 ; equatorial breadth 0'05, polar breadth 01. 



Habitat. Central Pacific, station 27l, surface. 



2. Diploconus cyathiscus, n. sp. (PL 140, fig. 3). 



Mantle of the double cone thin walled, not compressed ; its transverse section circular ; its 

 contour strongly convex ; its surface with six stronger and many smaller ribs ; its margin with 

 numerous, straight and long, parallel denticles. Diameter of its mouth half as long as the whole 

 shell and one and a half times as long as its equatorial diameter. The two principal spines 

 one-fourth longer than their conical sheath. The eighteen smaller spines about half as long, thin, 

 straight. Both geotomical spines thick and short. 



Dimensions. Length of the shell O2 ; equatorial breadth O07, polar breadth O'l. 



Habitat. West Tropical Pacific, Station 225, surface. 



3. Diploconus cylindrus, n. sp. 



Mantle of the double cone thick walled, dark, cylindrical, not compressed ; its transverse 

 section circular ; its contours straight, parallel ; its surface with strong parallel straight longitudinal 

 ribs ; its margin irregularly dentatecl. Diameter of its mouth one-fifth as long as the whole shell 

 and two-thirds as long as the diameter of the equatorial intumescence. The two principal spines 

 prismatic, nearly twice as long as their cylindrical sheath. The smaller spines about half as long, 

 thin, conical, straight. (Eesembles the medial part of Hexacolpus trypanon, PI. 140, fig. 11.) 



Dimensions. Length of the shell 0'25 ; equatorial breadth O08, polar breadth 0'05. 



Habitat. North Pacific, Station 241, surface. > 



