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



externally Axinus angulatus appears congeneric, it is possible that the hinge may be 

 differently constructed. 



Cryptodon watsoni, n. sp. (PI. XIV. figs. 1-la). 



Testa tenuis, paulo insequilateralis, anguste lunulata, mediocriter globosa, rotundata, 

 superne lata, subquadrata, alba, incrementi lineis tenuibus (hie illic fortioribus) striata, 

 striis minutis radiantibus irregularibus sculpta. Margo dorsi anticus parum declivis, 

 leviter concavus, posticus rectiusculus, fere horizontalis. Latus anticum aliquanto 

 augustatum, obtuse subnasutum, posticum multo altius, late curvatus. Margo ventris 

 regulariter curvatus, antice oblique ascendens. Umbones parvi, mediocriter acuti, 

 paulo antemediani, parum supra marginem producti. Cardo edentulus. Ligamentum 

 elongatum, angustum, in sulco pone umbones intra marginem positum. Pagina interna 

 radiatim substriata, antice lineis elevatis paucis radiatim curvantibus instructa. 

 Cicatrices intequales, postica subpyriformi, antica irregulari, obliqua, partim intra 

 lineam pallii producta. 



This species is white, moderately convex, rather thin, broad above, somewhat 

 shouldered on both sides, curved beneath. It is much higher behind than in front, 

 where, through the upcurving of the ventral margin, it has a faintly nasute appearance. 

 The valves are slightly inequilateral, the beaks, which are small, only a little elevated 

 above the dorsal line and not very acute, being situated somewhat in advance of the 

 middle. They also present a narrow lunule which is elevated along the centre, then 

 somewhat depressed. The sculpture consists of fine concentric lines of growth, some of 

 which, at intervals, are more conspicuous than the rest, as if marking periodical cessation 

 (^f orowth. In addition the surface exhibits a fine but rather irregular radiate striation. 

 The front dorsal margin is very slightly concave and a trifle oblique. The posterior is 

 almost rectilinear and nearly horizontal. The ventral outline is regularly curved 

 and much ascending anteriorly. The hinge is toothless and the ligament must be 

 only partially visible externally, being contained in a narrow groove within the hinder 

 dorsal margin, of which it occupies rather more than half the length. The inner surface 

 of the valves is not particularly glossy, radiately substriate, and marked in front 

 with one or two slender elevated lines curving; from the beaks to the anterior muscular 

 scar. This is of an irregular elongate form, almost parallel with the ventral margin, 

 and falls partly within the simple pallial line. The hinder scar is perhaps a trifle larger 

 and somewhat pear-shaped. 



Length 20 mm., height 17^, diameter 11. 



Habitat. — Station 219, off the north of the Admiralty Islands, at a depth of 

 150 fathoms; coral mud. 



