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



dextra polita, concentrice exilissimc striat;i, striisquc radiautibus obsolctis sculpta ; 

 sinistra iiitida, concentrice minuta lamellata, minutissimequc radiatim striata. Auricuht' 

 mininioe, subseciiuiles. Pagina interna nitida, lii'is aJl)is tenuibus denis instrncta. 



This species is rather higher than hmg, romuloil, Imt narrowed and produced above. 

 It is very thin, slightly inec[uivalve, gaping considerably on both sides, glossy, pellucid 

 around the outer margin, and rather thicker and more opaque towards the beaks. The 

 right valve is the deeper, somewhat more shining than the left, exhibiting very fine line;^ 

 of growth and indistinct radiating substriations. The left valve is more strongly concentri- 

 cally sculptured, and the radiating stride are more dense and stronger'. The auricles are 

 very small and almost of the same size. The beaks are acute, forming an apical angle of 

 about 105 degrees, the converging dorsal lines being sliglitl}- concave. The interii)r of the 

 valves towards the umbones, and rather more than half way across the valves, is more 

 opaque than exteriorly, and is strengthened by ten white slender brae of different lengths. 

 The outer pellucid zone of the right valve is very prettilj' blue-prismatic within. 



Length 20 mm., heioht 22, diameter 4i. 



Habitat. — Station 207, west of the island of Luzon, Philippines, at a depth of 700 

 fatlioms ; blue mud. 



The chief characteristics of this species Ijesides form, are its fragility, fine sculpture, 

 glossy surface, and the ten radiating costellse. The right vnlve is somewhat yellowish, 

 and the left whiter. 



AmussiiDii jejfrei/sii, n. sp. (PI. XXIII. figs. 2-2t). 



Testa tenuissima, paulo inajquivalvis, oldique rotunde ovata, utrinque conspicue hians, 

 subnitida, valvis dissimiliter sculptis, divcrseque coloratis dextra superru' in medioque 

 aurantiaca, marginem versus pallida diaphaua, radiatim teuuiter lirata, incrementique 

 lineis ornata, valva sinistra superne in medioque quam dextra pallidiore, inferne pellucida, 

 concentrice tcnuiter lirata. Auriculae parvas, subajquales, ad latera obliquae, postica 

 valvse sinistrse cseteris leviter dissimile. Pngina interna nitens, liris radiantibus 

 insequalibus albis abrupte terminatis circiter 15 instruct;!. 



This very beautiful species is higher than long, compressed, gapes considerably 

 at the sides, is slightly oblic^ue, roundl}- ovate, and a little inec[uivalve. The valves are 

 very thin, somewhat differently coloured, and ornamented with different sculpture. The 

 deeper valve (regarded as the right) is more brightly coloured, having the upper and 

 central portion tinted bright orange, and the outer l)order very much paler and 

 transparent. It is sculptured with numerous fine rounded lirse which radiate from the 

 beak and extend almost to the circunifercnee all round. In addition it is marked with 



