1000 THE VOYAGE OF H.M.S. CHALLENGER. 



only two of the former are visible, and represent the figure of a vertical 8 ; seen from 

 the sagittal poles, only two of the latter are visible, in the form of a larger horizontal co . 

 Octotympanum may be derived from Acrocuhus by reduction of the lateral frontal ring 

 to a knot (or cross-point), and by maximal extension of the two parallel horizontal rings, 

 which become curved downwards laterally and touch at the lateral poles. 



1. Octotympanum octospinum, n. sp. 



Mitral gates smaller than the basal gates ; upper halves of the two lateral meridional rings 

 smaller than the lower halves. Emgs with few scattered thorns ; four larger simple spines, nearly 

 horizontal in the equatorial plane ; two on the poles of the sagittal axis, two on the poles of the 

 lateral axis. Four other large spines, nearly vertical at the four lowermost basal points of the sub- 

 frontal rings. 



Bimcnsmis. — Length of the shell 0'09, breadth 0'12. 



Habitat. — Central Pacific, Station 273, depth 2350 fathoms. 



2. Octotympanum octonarium, n. sp. (PI. 94, fig. 3). 



Mitral gates of the same size as the basal gates ; upper and lower halves of the meridional rings 

 of equal size. Eiugs armed with numerous stout, thorny, simple and forked spines ; four very large 

 branched spines in the equatorial plane, nearly horizontal, two on the poles of the sagittal, two on 

 the poles of the transverse axis, the latter curved downwards. 



Dimensions. — Length of the shell O'l, breadth 0'2. 



Habitat. — Central Pacific, Station 271, depth 2425 fathoms. 



3. Octotympanum arborescens, n. sp. 



Mitral gates smaller than the basal gates, the lower halves of the lateral rings being broader 

 than the upper halves. Kings very spiny, armed with numerous branched spines ; ten \-ery large 

 arborescent spines on the lateral rings ; two on the poles of the transverse axis, eight on the pro- 

 minent corners of the cUagonal axes (four upper and four lower) ; sagittal ring thorny. 



Dimensions. — Length of the shell 0'8, breadth 0"16. 



Habitat. — Central Pacific, Station 274, depth 2750 fathoms. 



4. Octotympanum cervicorne, n. sp. (PI. 94, fig. 2). 



Mitral and basal gates of equal size, the lower and upper halves of the two crossed lateral rings 

 being nearly equal. Eings armed with numerous branched spines ; two very large spines, similar 

 to the antlers of a deer, on the poles of the lateral axis ; thek branches about half as large as the 

 whole shell. Sagittal ring nearly smooth, with few small thorns on the poles of the main axis. 



Dimensions. — Length of the shell 0'16, breadth 0-2. 



Habitat. — Central Pacific, Station 272, depth 2600 fathoms 



