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



ridge down the middle from the apex to the lower extremity, and a more distinct keel 

 nearer the dorsal mari,nn, marking oft' a smooth upper area. The front dorsal margin is 

 veiy oblique, somewhat curved, and shorter than the jjosterior, which is less sloping and 

 rather concave. The ventral outline is widely arcuate in front and at the middle, but 

 shallowly sinuate at the depression. The umbones are small, only slightly raised above 

 the hinge-line, hardly incurved, and, at the extreme tip, do not appear to incline either 

 anteriorly or posteriorly, although, when the valves are viewed externally, they curve 

 over towards the hinder .side. The right valve has a short Imt very erect lamellar 

 lateral tooth on each side near the umbo, the posterior, however, is prolonged under 

 the dorsal margin, in the form of a ridge, some distance along the rostrum. It also has, 

 just in front of the apex, a small triangular dentiform projection of the dorsal margin. 

 The left valve has no lateral, but a single thickish tubercular denticle, just in front of 

 the apex. The ligament is small, oblique, and posteriorly inclined, not in a prominent 

 process, but attached to the inner surface of the shell. The interior is glossy, exhibiting 

 more or less of the external concentric ribbing, especially in the right valve. The 

 muscular impressions are indistinct. 



Length 6 mm., height 3f, diameter 2. 



Habitat. — Port Jackson, Sydney, in 2 to 10 fathoms (Challenger) ; Moreton Bay 

 (British Museum). 



Section K. 



Necera clavicidata, Dall (PI. IX. figs. 8—8/'). 



Necnra daviailala, Dall, Hull. Mus. Comp. Zoul., vol. ix., Xo. 2. p. 11 2. 

 Habitat. — Station 33, oft" Bermuda, in 435 fathoms ; coral mud. 



None of the Challenger specimens are as large as that descriljed by Dall from 

 Station 44 of the " Blake " Expedition, the largest being only 6 mm. in length. The 

 right valve has a singular acute (almost hooked) denticle just in front of the apex, the 

 left l^eing entirely without teeth. The great feature of this species is the peculiar shelf- 

 like expansion within the posterior dorsal margin, the special use of which is at present 

 a matter of conjecture. It may be for the reception of the hinder adductor muscle, 

 although I am inclined to believe such is not the case. 



Necera congenita, n. sp. (PL X. figs. 1-1/*). 



Testa NecercB davicidatw similis, sed angustior, minus convexa, postice latius 

 rostrata, margine dorsi postico rectiusculo, ventrali postice vix sinuato, clavicula interna 

 magis elongata, et ligamenti fossa angustiori instructa. 



