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



This is a solid, dirty whitish, little species, equUateral, fairly convex, of a 

 triangularly ovate form, rather sharply rounded in front, and acutely, but shortly, 

 rostrate behind. The valves exhibit a shallow radiating depression down the posterior 

 side, which is defined in front by a faint ridge, and causes a feeble emargination in 

 the otherwise regularly and broadly curved lower margin. The sculpture consists of 

 numerous strong concentric riblets, wdiich are furthest apart towards the beaks, and 

 gradually approximate as the shell increases. They become slender by degrees at the 

 sides, and in a great measure terminate at the rounded ridges which border the lunule 

 and area. In addition to the costse there ai-e two or three faint radiating irregular 

 lii'se between the libs which run from the umbones to the anterior end of the ventral 

 outline. The lunule is long, narrowly elliptical, and somewhat sunken, whilst the area 

 is much broader and sharply oval, both being ornamented with, fine radiating lirse, the 

 continuations of some of the concentric riblets. The dorsal margins are about equally 

 oblique, the anterior being nearly rectilinear, and the posterior a little concave. The 

 hinge-plate is rather strong, and furnished with about fifteen teeth on each side the 

 minute central cartilage -pit. Interior glossy, thickened. 



Length 6^ mm., height 5J, diameter 3i. 



Habitat. — Station 188, south of New Guinea, in 28 fathoms; green mud. 



In general appearance this species closely resembles Leda ventricosa of Hinds, but 

 may be distinguished from it by its smaller size, comparatively greater solidity, the 

 more distant riblets upon the umbones, the more distinct impressed ray, and the 

 absence of the puckers or nodules on the lunule. 



Leda neceriformis, n. sp. (PI. XX. figs. 2— 2a). 



Testa parva, elongata, postice anguste rostrata et biearinata, parum convexa, valde 

 insequUateralis, concentrice tenuiter striata. Margo dorsi anticus brevis, valde curvatim 

 declivis, posticus elongatus, minus obliquus, concavus. Margo ventris antiee et in 

 medio late arcuatus sub rostrum leviter incurvatus. Dentes ad triginta, insigniter cavi. 

 Pagina interna nitida, per medium rostri porcata. 



This species is elongate, slenderly beaked behind, rather compressed, very 

 inequilateral, white and sculptured with fine concentric strise. From the beaks two 

 keels run to the rather squarely truncate end of the rostrum, the space between them 

 and between the upper one and the dorsal margin being concave. The front dorsal 

 slope is short, very much curved and very oblique, the posterior, on the contrary, being- 

 long, concave, and less sloping. The anterior end is sharply rounded, the lower outline 

 broadly arcuate in front and along the middle, but feebly incurved at the rostrate 

 extremity. The umbones are small, and situated at about two-fifths of the total length 



