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



equal to more than half the length ; anterior extremity ol >liquely rounded, posterior 

 truncated, produced below the middle into a beak-like or irregularly squamous process ; 

 dorsal margin highest over the hinge-tubercle, thence sloping with a sinuous curve 

 backwards, and ending in a well-marked angle, ventral margin nearly straight ; seen 

 from above, the outline is oblong and subhexagonal, with parallel sides, which converge 

 gently towards the front, abruptly and almost at a right angle behind ; anterior 

 extremity wide, subtruncated, and having a small centi'al mucro, posterior produced in 

 the middle into a broad protuberance ; end view vaulted, dorsal margin arched, ventral 

 Inroad and only slightly convex. Surface of the valves marked with small, shallow 

 pittings, which are rather irregularly disposed and obscurely rounded. Length, l-50th 

 of an inch ("5 mm.). 



Found in a dredging from a depth of 7 fathoms off Ascension Island ; the single 

 valve shown in figures e-h is from Balfour Bay, Kerguelen Island, but it may well be 

 doubted whether it really belongs to this species. The type specimens were from 

 Mauritius and Colon-Aspinwall. 



[PL XV. fig. 7, cir-li. a Shell of Ascension Island specimen seen from left side, 

 h from above, c from below, d from front ; figures e-h exhibit similar views of a right 

 valve from Balfour Bay. Magnified 60 diameters.] 



39. Cythere curvicostata, n. sp. (PL XII. fig. 4, a-d). 



Carapace compressed oblong ; seen from the side, subclavate, rather higher in front 

 than behind, greatest height not so much as half the length ; anterior extremity well 

 rounded and bordered with a regular row of small teeth, which extend a short distance 

 along the ventral margin ; posterior subtruncated, slightly produced below the middle, 

 the produced portion divided into teeth similar to those of the anterior extremity ; dorsal 

 margin highest in front, and falling by two abrupt but shallow steps towards the posterior 

 extremity ; ventral margin straight. The lateral surfaces of the sheU exhibit two or 

 three sinuous longitudinal ribs extending nearly the whole length of the valves, and 

 towards the margins some irregularly-disposed smaller ribs ; the interspaces are occupied 

 by small fossse closely set and arranged in longitudinal rows ; seen from above, the 

 outline is oblong, thrice as long as broad, nearly equal in width throughout, the sides 

 parallel, the extremities broad and subtruncate ; end view subquadrangular, height 

 greater than the width. Length, l-45tli of an inch ("53 mm.). 



One or two specimens only in a dredging from near Booby Island, in a depth of 6 to 

 8 fathoms. In style of surface ornament as well as in general shape the species is not 

 very unlike the British Cythere emaciata, but a critical examination shows numerous 

 important differences. Yet the peculiar disposition of the rib-work and associated fossae, 

 the finely-dentated margins and fan-like posterior expansions of the valve-margins 

 suggest either a community of descent, or (which is scarcely likely) exposure to con- 



