140 



THE VOYAGE OF H.M.S. CHALLENGER. 



ventral very slightly convex ; seen from above, the outline is ovate, widest in the middle, 

 the greatest width being fully equal to the height of the shell, the lateral margins are 

 regularly and evenly convex, the extremities broad and subtruncate, with a bimueronate 

 central projection, that of the posterior extremity being very large and stout ; end view 

 triangular, sides convex, base straight, with a deep sinuation on each side of the middle, 

 basal angles rounded and slightly produced, apex much produced and subacute. The 

 general surface of the shell is smooth, the sides evenly convex, the ventral surface 

 flattened and irregularly nodulated ; within the ventral margin of each valve, and along 

 nearly its whole length, runs an elevated crest, which is pierced by numerous (20 to 30) 

 circular foramina. Length, l-22d of an inch (1"1 mm.). 



Of this fine and very distinct species several specimens were obtained ofi" Christmas 

 Harbour, Kerguelen Island, in a depth of 120 fathoms (Station 149) ; also at Station 335, 

 north of Tristan d'Acunha, lat. 32° 24' S., long. 13° 5' W., 1425 fathoms. 



[PL XXXIV. fig. 6, a-d. a SheU seen from left side, h from above, c from below, 

 d from front. All magnified 50 diameters.] 



9. Cytheropteron mucronalatum, n. sp. (PL XXXIII. fig. 8, a-d). 



In form very similar to the preceding species, but more robust and higher in propor- 

 tion to its length, and devoid of the fenestrated alseform ridge ; seen from the side, 

 broadly subovate, height equal to more than two-thirds of the length ; anterior extremity 

 broadly rounded, irregularly and roughly dentate, posterior narrower, but not produced, 

 and bearing a few blunt spines ; dorsal margin very boldly arched, not flattened, highest 

 in the middle, ventral gently convex ; seen from above the outline is ovate, widest in the 

 middle, width less than the height ; the sides are irregularly convex, and converge 

 gradually towards the front, but more abruptly behind, both extremities running out 

 into broad obtusely mucronate projections ; end view triangular, the sides only slightly 

 convex, and much longer than the base, which is indented in the middle, angles acute ; 

 the general surface of the shell is smooth, sides evenly convex, ventral suface almost flat, 

 except for a central longitudinal depression ; within and parallel to the anterior and 

 ventral margins of the valves runs an elevated ridge, which terminates not far from the 

 posterior extremity of the shell in a strong, but not very long spinous projection. 

 Length, l-20th of an inch (1*3 mm.). 



This species, though nowhere abundant, occurred in several dredgings — 



From this list it appears that Cytherojiteron muoronalatum is an inhabitant exclusively 



