1891.] THE 'challenger' expedition. 443 



longitudinis siti, medwcriter acuti ; lumila elongata, lanceolata, 

 in medio prornitiens, porca distincta circumdata ; area dorsi 

 postica hand definita, in medio prominens ; linea cardinis 

 mediocriter valida, dentihus novenis posterioribus septemque 

 anteriorihus acutis instructa. Fossa lir/amenti parva, profunda, 

 postice ohliqwx ; pagina interna aJbo-margaritacea, radiatim 

 minute sid>stnata, margine simplice circumdata. 

 Longit. 5| millim., alt. 4j, diam. 3. 



24. NucuLA UMBONATA. (Plate XXXV. fig. 24.) 



Testa parva, incequilateralis, trigono-ovata, nitida, epidermide 

 dilute lutescenie incluta, apicem, versus pallidior, incrementi 

 lineis striisque radiantibus tenuissimis sculpta ; margo dorsi 

 utrinque valde declivis, purum arcuatus, ventris late cw'vatus ; 

 latus anticum ad extremitatem subangidatum, posticum acute 

 curvatum ; umbones promi.nentes, pallidi, ante medium collocati ; 

 linea cardinis valida, dentibus anteriorihus series novemque 

 posterioribus instructa. Margo valvarum inferior intus minute 

 denticidatus. 

 Longit. 3j millim., alt. 3, diam. 2. 



This species is peculiar for its somewhat triangular form, pro- 

 minent 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, subjjellucida, cfquivalvis, paulo ohliqua ; valves costis 

 tenuibus subdistantibus plicisque concentricis paucis subcancel- 

 latce, undique minute radiatim microscopice striatce. ; casta tenues, 

 interdum palUde lutescentes, margines versus plus minus minute 

 squamatce ; auricidce valvce sinistra incequales, antica majori ; 

 postica valvce decctrce liris sex confertim squarnatis instructa, 

 inferne mediocriter prof uncle sinuata ; pagina interna nitens, 

 radiatim liaud profunde sulcata, sulds liris exte^'nis 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 radi- 

 ating 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. 



