672 THE VOYAGE OF H.M.S. CHALLENGEE. 



The geuus Tholoma (PL 10, figs. 10, 13) diifers from the preceding Tholostaurus 

 (its probable ancestral form) only in the duplication of the shell. The outer (secondary) 

 shell has the same cross-form as the inner (primary) shell. In the two observed species 

 the growth seems to be different, in tlie first species all four cupolas of each cross 

 being of the same size, form, and age ; in the second two opposite cupohxs, larger and 

 apparently older than the other two. Tholoma is possibly the oft'spring of Stauro- 

 tholonium, from which it may have been produced by loss of the medullary shell. 



Subgenus 1. T/wlomantha, Haeckel. 

 Definition. — Surface of the sliell smooth, without radial spines. 



1. Tholoma quadrigemimim, n. sp. (PL 10, fig. 10). 



All four chambers in each cortical shell nearly equal, subregular. Surface smooth, without 

 radial spines. Structure of the network in both shells similar. Pores regular, circular, twice as 

 broad as the bars ; six to eight in the basal semicircle of one inner, ten to twelve in the semicircle 

 of one outer cupola. 



Diinensions. — Diameter of the outer shell 014, of the inner 0-09 ; pores 0-006, bars 0'003. 



Habitat. — South Pacific, Station 302, surface. 



Subgenus 2. Tholomura, HaeckeL 

 Definition. — Surface of the shell with radial spines. 



2. Tlioloma metallasson,^ n. sp. (PL 10, fig. 13). 



Two opposite chambers in each cortical shell larger than the two others ; the larger chambers 

 of the inner shell corresponding to the smaller chambers of the outer shell. Network in both shells 

 of the same structure, regular, with circular, hexagonally framed pores of the same breadth as the 

 bars ; eight to ten in the semicircle of an inner, fourteen to sixteen in the semicircle of an outer 

 cupola. Numerous (twenty to thirty or more) thin, cylindrical, bristle-shaped, long radial spines, 

 symmetrically disposed. 



Dimensions.— Maiov axis of the outer shell 0'26, minor axis 0'2 ; major axis of the inner 

 shell 016, minor axis 013 ; pores and bars 0'006. 



Habitat. — Pacific, central area, Station 265, depth 2900 fathoms. 



Genus 295. Staurotholiis,'^ n. gen. 



Definition. — Tholonida with simple cortical shell (without external veil), 

 composed of four hemispherical cupolas in cross-form, opposite in pau's on the poles of 



' Alternating, inTaXKaaouu. ^ SteMroi/wtes = Cupolas cross- wise disposed ; utiuv^o;, ioKui. 



