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



The genus Tholartus (PI. 10, fig. 1) represents the most simple form of all 

 Tholonida, a three-jointed simple cortical shell without medullary shell ; two hemi- 

 spherical cupolas or domes are attached on both lateral sides of a simple triaxial 

 ellipsoidal central chamber, corresponding to the poles of its lateral or transverse axis. 

 Tholartus may be regarded as the common ancestral form of all Tholonida, and may be 

 derived either from the Pylonida Monozonium (by complete lattice -locking of the 

 lateral wings) or from the Ellipsida Cenellipsis (by development of two lateral 

 protuberances on the poles of the transverse axis). But it is also possible that the 

 absence of the medullary shell is produced by reduction, and in this case Tholartus may 

 be descended from Amphitholus. 



Subgenus 1. Tholartella, HaeckeL 

 Definition. Surface of the shell smooth or rough, without radial spines. 



1. Tholartus tricolus, n. sp. (PI. 10, fig. 1). 



Central chamber one and a half times as high and as broad as both cupolas. Surface of the 

 shell rough. Pores regular, circular, without hexagonal frames, three tunes as broad as the bars ; 

 about sixteen on the half meridian of the central chamber (or its vertical diameter). 



Dimensions. Length of the shell (major axis of the central chamber, vertical) 01, breadth of 

 the shell (major axis of the whole three-chambered shell, horizontal) 0'13 ; pores O'Ol, bars - 003. 



Habitat. Pacific, central area, Station 270, depth 2925 fathoms. 



2. Tholartus paniscus, n. sp. 



Central chamber of the same breadth, but of the double height of both cupolas. Surface of 

 the shell rough. Pores regular, circular, with hexagonal frames, of the same breadth as the bars ; 

 eight to ten on the half meridian of the central chamber. 



Dimensions. Length of the shell (major axis of the central chamber, vertical) 012, breadth of 

 the shell (major axis of the whole three-chambered shell, horizontal) 015 ; pores and bars O'OOS. 



Habitat. Pacific, central area, Station 271, depth 2425 fathoms. 



3. Tholartus isocolus, n. sp. 



Central chamber of the same breadth and height as both cupolas. Surface of the shell smooth. 

 Pores irregular, roundish, once to three times as broad as the bars ; ten to fifteen on the half 

 meridian of the central chamber. 



Dimensions. Length of the shell Oil, breadth 014 ; pores 0'004 to O'Ol, bars O'OOS. 



Habitat. Pacific, central area, Station 270, depth 2925 fathoms. 



