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



less distinctly radiately substriated. The pallical sinus is broadly curved, and reaches 

 about one-third of the length across the valves. 



Length 6 mm., height 5, diameter 3. 



Habitat. — Station 185b, east of Cape York, North Australia, at a depth of 

 155 fathoms; coral sand. 



Young shells do not show so much tendency to posterior rostration as those more 

 adult. 



Montctcuta cylindracea, n. sp. (PI. XII. figs. 4-46). 



Testa elongata, cylindracea, valde couvexa, insequilateralis, albida, utrinque rotundata, 

 increment! lineis tenuibus striata, epidermide tenui fibrosa prsecipue prope marginem 

 induta. Margo dorsi longior (anticus X) horizontalis, levissime arcuatus, brevior 

 (posticus ?) oblique curvatus. Margo ventralis late arcuatus utrinque ascendens. 

 Umbones subacuti, paulo elevati, circiter in \ longitudinis collocati. Pagina interna 

 alba, nitida. Cardinis margo aliquanto incrassatus, fossa obliqua" ligamentum continente 

 postice munitus. Cicatrix muscularis antica elongata, postica brevior, rotundata. 



This species is very elongate and very inequilateral. It is rather cylindrical, rounded 

 at both ends, a little wider in front than behind, and considerably swollen. It is 

 moderately strong for the genus, sculptured with fine concentric lines of groMi^h, whitish, 

 clothed inore or less, especially towards the outer margin, with a pale yellowish fibrous 

 epidermis, which is narrowly reflexed within the valves. The longer, or what I consider 

 the anterior dorsal margin, is horizontal and faintly excurved ; the posterior, being very 

 much shorter, descends obliquely and is somewhat arcuate. The lower outline is only 

 slightly convex, and ascends rather more abruptly behind than in front. The umbones 

 are moderately acute, a little raised above the hinge-line, and situated at about a fourth 

 of the entire length from the posterior extremity. The hinge-plate is slightly thickened, 

 especially behind the beaks, where there is a shallow oblique pit or socket for the recep- 

 tion of the ligament. The interior of the valves is white and a little glossy. The 

 anterior muscular scar is very long, the posterior shorter, broader, and rounder. 



Length 15 mm., height Sg, diameter 7. 



Habitat. — North Atlantic, in deep water. 



This species is remarkable on account of its cylindrical form, the convexity of the 

 valves, and its comparative solidity. 



Montaciita occidentalis, n. sp. (PI. XII. figs. 5-56). 



Testa transversim ovata, minuta, tenuis, valde insequilateralis, utrinque rotundata, 

 antice angustior, alba, nitida, incrementi striis minutis sculpta. Margo dorsi anticus 



