298 THE VOYAGE OF H.M.S. CHALLENGEK. 



8. Ellipsoxiphus atractus, n. sp. (PL 14, fig. 1). 



Proportion of the major axis to the minor = 3:2. Shell spindle-shaped, very thick walled, with 

 roundish, very irregular meshes, twice to five times as broad as the hars ; eight to twelve on the half 

 equator. The meshes are partly simple, oblong, partly lobed or composed of two to four (commonly 

 three) confluent meshes. Surface smooth. Polar spines very short and thick, shorter than the 

 minor radius of -the shell, three-sided pyramidal, with prominent, somewhat contorted edges. 



Dimensions. Longer axis O'l 5, shorter axis O'l ; meshes 0'007 to 0'02, bars 0'004; length of 

 the polar spines 0'04, basal thickness 0'03. 



Habitat. Central area of the Pacific, Station 268, depth 2900 fathoms. 



Genus 126. Axoprunum, 1 n. gen. 



Definition. E llipsida with simple ellipsoidal shell, the main axis of which is 

 prolonged at both poles into two opposite spines of equal size and similar form. "Within 

 the cavity of the shell four radial rods arise from its inner surface, two in the main axis, 

 two in the smallest axis, perpendicular to the former ; their free inner ends are at the 

 same distance from the centre. 



The genus Axoprunum possesses precisely the same shell as the foregoing Ellipso- 

 xiphus, but differs from it in a very remarkable peculiarity. The two polar spines are 

 centripetally prolonged into two internal beams, and perpendicular to these are two other, 

 opposite, transverse beams, marking the minor axis of the ellipsoid. The free inner ends 

 of all four radial rods bear little thickened knobs, and are at the same distance from 

 the centre. It therefore appears as though a central, spherical, medullary shell had 

 been lost, and this gives a strong support to the important hypothesis, that in many 

 Sphserellaria, where the medullary shell is absent, it may have been lost by 

 phylogenetic reduction or retrograde metamorphosis. In this case Axoprunum (and 

 Ellipsoxiphus) would arise from Lithatractus. 



1. Axoprunum stauraxonium, n. sp. (PL 48, fig. 4). 



Shell ellipsoidal, one and one-third times as long as broad, with smooth surface. Network 

 regular, with circular meshes four times as broad as the bars. Two polar spines three-sided pyramidal, 

 half as long as the shell, as thick at the base as a single mesh. Four inner radial beams (lying, two 

 in the major and two in the minor axis of the ellipsoid) very thin, at the central free ends knob-like, 

 thickened. The distance between two opposite beams equals one-third of the minor axis, and 

 indicates probably the diameter of the lost spherical medullary shell. (Three perfect and complete 

 specimens of the same size and shape were observed.) 



1 Axoprunum = Plum with axis ; <?, r^tvtir. 



