1891.] THE 'challenger' expedition. 443 



longitudinis siti, mediocriter acuti ; lunula elongata, lanceolata, 

 in medio prominens, porca distincta circumdata ; area dorsi 

 postica liaud definita, in medio prominens ; linea cardinis 

 mediocriter valkla, dentibus novenis posterioribus septemque 

 anterioribus aciitis instructa. Fossa Ikjamenti 'parva, profunda, 

 postice obliqtia ; pagina interna albo-margaritacea, radiatim 

 minute substriata, margine simplice circumdata. 

 Longit. 5^ millim., alt. 4^, diam,. 3. 



24. NucuLA UM BON ATA. (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, ptarum arcuatus, ventris late curvatus ; 

 latus anticum ad extremitatem subangulatum, posticum acute 

 curvatum ; umbones promlnentes, pallidi, ante medium coUocati ; 

 linea cardinis valida, dentibus anterioribus series novemque 

 posterioribus instructa. Margo valvarum inferior intus minute 

 denticulatus. 

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



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

 minent umbones, the fine radiating striss, 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, subpeUucida, cequivalvis, paulo obliqua ; valvoi costis 

 tenuibus subdistantibus plicisque concentricis paucis subcancel- 

 latce, undique minute radiatim microscopice striatce ; costai tenues, 

 interdum pallide lutescentes, margines versus plus minus ininute 

 squamatce ; auriculae valvce sinistr(B incequales, antica majori ; 

 postica valvce dextrce liris sex confertim squamatis instructa, 

 inferne mediocriter profunde sinuata ; pagina interna nitens, 

 radiatim Jiaud profunde sulcata, sulcis liris externis congruov- 

 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 strise 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. 



