13S THE VOYAGE OF H.M.S. CHALLENGER. 



The type-specimens of Cytheropteron intermedium, whicli differ scarcely in any 

 degree from those here described, were fossils from the Antwerp Crag (Tertiary). The 

 Challenger specimens are from Vigo Bay, 11 fathoms. 



[PL XXXIV. fig. 1 , a~d. a Shell seen from left side, h from above, c from below, 

 dhom front. Magnified 50 diameters.] 



5. Cytheropteron abyssorum, n. sp. (PL XXXIV. fig. 3, a-d). 



Shell tumid ; seen from the side, subrhomboidal, highest in the middle, height equal 

 to about two-thirds of the length ; anterior extremity obliquely rounded, posterior pro- 

 duced into a median acuminate beak ; dorsal margin moderately arched, sloping gently 

 towards the front, and very steeply behind, ventral strongly convex ; seen from above, 

 the shape is irregularly hexagonal, widest behind the middle over the alar prominences, 

 width a little greater than the height ; from the alar processes the lateral margins 

 converge gently and with a slight sinuation towards the front for about one-third of the 

 length of the shell, then converging abruptly, terminate anteriorly in a large acuminate 

 process ; behind the alse the sides converge for a short space rectangularly, then are 

 directed backwards, meeting at an acute angle in a long acuminate projection ; end-view 

 triangular, apex acute, lateral angles broad and produced ; lateral margins gently 

 convex, ventral almost straight. Surface of the valves marked partially on the sides 

 with angular excavations, and on the ventral surface with irregular longitudinal sulci. 

 Length, l-50th of an inch ('5 mm.). 



A very well-marked species, if we exclude the possibility of its being a sexual form 

 or the young of that next to be described, Cytheropteron assimile, to which it bears 

 considerable resemblance. Cytheropteron assimile, however, is much larger, the contours 

 much more rounded, and the lateral alse not so prominent as in Cytheropteron ahyssorum. 

 Dredged in lat. 42° 42' S., long. 134° 10' E., 2600 fathoms (Station 160). 



[PL XXXIV. fig. 3, a-d. a Shell seen from left side, h from above, c from below, 

 cZfrom front. Magnified 50 diameters.] 



6. Cytheropteron assimile, n. sp. (PL XXXIV. fig. 3, a-d). 



Shell, seen from the side, oblong, flexuous, subrhomboidal, not much lower in front 

 than behind, highest near the middle, the height being equal to more than half the 

 length ; anterior extremity evenly rounded, posterior produced in the middle into a wide, 

 obtusely rounded beak, above which it is rather deeply excavated, nearly straight below 

 and looking obliquely backwards and downwards, dorsal margin boldly arched, ventral 

 convex behind and almost straight in front ; seen from above, almost like Cytherop)teron 

 ahyssorum, but narrower in proportion to the length ; the end view is much broader and 

 more rounded dorsally than in CytheropMron abyssorum, while the ventral surface is 

 rather deeply indented, and the angles, instead of being produced, are rounded off. The 



