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



lustre, and sculptured with fine lines of growth. The valves exhibit a broad shallow 

 depression, radiating from the umboues and marking ofi' the rostrum. This is rather 

 abruptly truncate at the end, and faintly ridged from the beak to the lower corner, with 

 the lines of growth rather strongly developed. In the left valve there is a second slight 

 ridge, very close to the upper margin, marking off a linear dorsal area. The front dorsal 

 margin rises a little, close to the umbo, then descends obliquely with only a very slight 

 curve. The j^osterior margin is longer, slopes but very little, and is the least concave. 

 The ventral outline forms under the main portion of the shell a regular broad curve, but 

 at the termination of the radiating depression is gently sinuated. The umbones are only 

 a little elevated, and somewhat in advance of the centre. The left valve is toothless, but 

 possesses a minute and slightly oblique cartilage-pit just under the apex. The right valve 

 has a single slender elongate lamellar tooth on the posterior side, and the hinge-line in both 

 valves in front is a little expanded. The muscular impressions are very feebly indicated. 



Length 13 mm., height 7, diameter 5. 



Habitat. — Station 320, otf the mouth of the Eio de la Plata, in 600 fathoms ; green 

 sand. 



In form this species resembles to some extent Necera ohesa, Loven, and Necera 

 Umatilla, Dall. It is less convex than the former, is more sharply rounded in front, and 

 has less prominent umbones, the latter being more strongly sculptured. 



Necera herguelenensis, n. sp. (PI. XXIV. tigs. 8-8/*). 



Testa parva, subglobosa, postice bi'eviter rostrata, concentrice lirata, liris tenuibus 

 valde elevatis, sujara rostrum flexuosis, ad liram obliquam medianam obsoletis. Margo 

 dorsi anticus rectiusculus, declivis, posticus paulo concavus, minus obliquus. Margo 

 inferior antice et in medio late curvatus, sub rostro distincte sinuatus. Umbones parvi, 

 circa in medio, ad cacumina leves. Valva dextra dente unico valido submarginali pone 

 umbonem munita, valva sinistra contra edentida. 



This little shell is rather globose, nearly equilateral, rounded in front, much contracted 

 and beaked behind. It is white, and ornamented with about twenty, fine, much elevated, 

 concentric, lamelliform Yivsd, which are attenuated and flexuous in a depression or con- 

 striction marking off the rostrate end, and almost terminate at a slight keel which runs 

 from the umbones to the lower corner of the curved truncate beak. Above this carina 

 there is a second finer one, but nearer to the dorsal edge. The front dorsal margin is 

 somewhat oblique, but not much curved, the posterior being rather concave. The ventral 

 outline is distinctly sinuate under the rostrate end, and greatly arcuate at the lower part 

 and in front. The umbones are small, nearly central, and smooth at the tips. The hinge 

 is composed of a single strongish tooth in the right valve, situated within the dorsal 

 margin just behind the umbo. 



