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Show 1891.] THE 'CHALLENGER' EXPEDITION. 443 longitudinis siti, mediocriter acuti ; lunula elongata, lanceolata, in medio prominens, porca distincta circumdata; area dorsi poslica haud definita, in medio prominens ; linea cardinis mediocriter valida, dentibus novenis j^osterioribus septemque anterioribus acutis instructa. Fossa ligamenti parva, profunda, postice obliqua; pacjina interna albo-margaritacea, radiatim minute substriata, margine simplice circumdata. Longit. 5^ millim., alt. 4^, diam. 3. 24. NUCULA UMBONATA. (Plate XXXV. fig. 24.) Testa parva, incequilateralis, trigono-ovata, nitida, epidermide dilute lutescente induta, apicem versus pallidior, incrementi lineis striisque radiantibus tenuissimis sculpta ; margo dorsi utrinque valde declivis, parum arcuatus, ventris late curvatus ; lotus anticum ad extremitatem subangulatum, posticum acute curvatum ; umbones prominentes, pallidi, ante medium collocati ; linea cardinis valida, dentibus anterioribus series novemque posterioribus instructa. Margo valvarum inferior intus minute denticulatus. Longit. 3j millim., alt. 3, diam. 2. This species is peculiar for its somewhat triangular form, prominent umbones, the fine radiating striae, and the denticulated inner margin of the valves. The lunule is not clearly defined, but the posterior dorsal area is narrow and bounded by a slight but distinct ridge. 25. PECTEN CHALLENGERI. (Plate XXXV. fig 25.) Testa tenuis, subpellucida, cequivalvis, paido obliqua ; valvce costis tenuibus subdistantibus plicisque concentricis paucis subcancel-latce, undique minute radiatim microscopice striatce ; costce tenues, interdum pallide lutescentes, margines versus plus minus minute squamatce ; auricidce valvce sinistra? incequales, antica majori ; postica valvce dextrce liris sex confertim squamatis instructa, inferne mediocriter profunde sinuata ; pagina interna nitens, radiatim haud profunde sulcata, sulcis liris externis congruen-tibus. Longit. 18 millim., alt. 19, diam. 4|. Of this pretty species there is one perfect specimen and a single right valve. The substance of the shell is very thin, semipellucid towards the umbones, and rather more opaque elsewhere. The radiating striae are excessively minute, and a trifle more conspicuous in the right valve than in the left. The valves are decidedly oblique towards the front and smooth at the apices, the costae commencing at a distance of about two millimetres from the extreme tip. Most of the ridges in the left valve have a pale luteous tint upon their lower halves, a feature not present in the other valve of the same specimen, which, however, is peculiar in having a bright reddish streak on the dorsal slopes within. The few concentric plicae which produce the somewhat cancellated aspect of the exterior appear to be periodic marks of growth. |