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



both shells are extracapsular or cortical shells, therefore the inner shell of the latter 

 corresponds to the outer of the former. The inner spherical shell of all Diplosphserida 

 is composed of very delicate beams and large pores, which are either regular hexagonal 

 or irregular polygonal (never roundish). From its surface arise a variable number 

 (twenty to thirty) of stout long radial spines, which are invariably longer than the 

 shell diameter (often two to three times as long or more), and of three-sided prismatic 

 form, the three edges either smooth or serrate, often with three rows of lateral branches 

 (commonly three to five branches in each row) ; the latter are invariably of the same 

 form, coucavely curved towards the spine, and decrease in size towards the distal end. 

 From the three edges of each main spine in all Diplosphserida, at equal distances from 

 the centre, arise six very thin, thread-like lateral branches (a pair from each edge) 

 and connect the spine in a tangential direction with all neighbouring spines. In 

 this manner the polyhedral outer shell is formed, the meshes of which therefore 

 are always very large and triangular. Sometimes each of these primary triangular 

 meshes becomes filled up with a secondary network, either of regular quadrangular or of 

 irregular polygonal secondary meshes. Besides the constant twenty to thirty large 

 main spines, in the majority of Diplosphaerida bristle-shaped radial by-spines arise, 

 either from the inner shell (Diplosphoera) or from the outer (Drymosphfera), or from 

 both (Astrosphcera}. They are absent only in Leptosphcera. Commonly the by- 

 spines are simple, rarely forked or branched. The central capsule in the Diplosphee- 

 rida is usually enclosed in the inner shell ; often it completely fills up the latter, or 

 drives out a csecal protuberance through each mesh ; but these processes rarely unite 

 outside. The average size of the Diplosphserida, which are all pelagic organisms, is much 

 larger than that of the other Haliommida. 



Subgenus 1. Leptosphwrella, Haeckel. 

 Definition. Radial spines simple, without lateral branches. 



1. Leptosphcera hexagonalis, n. sp. (PL 19, fig. 2). 



Inner shell with regular, hexagonal meshes, and very thin, thread-like bars ; outer shell 

 twice as broad, with simple triangular meshes, liadial spines with three smooth edges. (Fig- 

 '2 represents the central capsule with numerous club-shaped saccules, prominent externally 

 through the meshes ; in the centre a large simple spherical .nucleus, one-third as broad 'as the 

 capsule. The skeleton of this species is identical with that of Diploapkcera liexagonalis, PI. 19, fig. 3, 

 but has no by-spines.) 



Dimensions. Diameter of the outer shell 0'3, of the inner O'lfi. 



Habitat. Cosmopolitan ; Mediterranean (Corfu), North Atlantic (Canary Islands), Tropical 

 Pacific, surface. 



