•280 THE VOYAGE OF H.M.S. CHALLENGER. 



tubercular tooth in each valve, and I have never met with any specimen having douljle 

 this number as is stated to be the case by JNIessrs. Adams and Chenu. Sometimes these 

 tubercles may be wanting in one or even in both valves, the latter being apparently a 

 rare occurrence. 



Modiolarca kerguelensis, n. sp. (PL XVI. figs. 8-8a). 



Testa fragilis, alba, aliquanto iridesceus, incrementi lineis striata, elongata antice 

 angustata, jjostice altior, curvatim oblique truncata, mediocriter convexa. Margo dorsi 

 posticus perelongatus, fere rectilinearis, rotund o-angulatim cum latere postico junctus. 

 Margo ventris antice indistincte subsinuatus, post medium late curvatus leviterque 

 adscendens, acute rotundatim in lateralem ambiens. Latus anticum brevissimum, sub- 

 acutum, posticum longissimum, latum. Umbones mediocriter prominentes, ad apicem 

 obtusi, circiter in I longitudiuis positi. Pagina interna paulo nitida. Cicatrix antica 

 prsecipua, subprofuuda, elongata, postica indistincta. Dentes duo cardinales in utraque 

 valva, anterioribus majoribus. Ligamentum internum elongatum, angustum, ligamento 

 externo perelongato, marginali. 



This species is oblong, trapezoidal, Ijroader behind than in front, xevy thin, white, a 

 little glossy and iridescent, and sculptured with very fine concentric lines of growth. 

 The valves are moderately convex, verj' inequilateral, the umbones, which are small, 

 only a little prominent, and terminate in an obtuse smooth gloss}^ boss, being situated far 

 in front, at about one-sixth of the whole length from the anterior extremity. The 

 anterior dorsal margin is short, nearly straight, and very oblique, forming, with the up- 

 curving lower margin, a sharply rounded (almost beaked) end. The hinder margin is long, 

 subrectilinear, and directed slightly upwards. The ventral outline is broadly arcuate, with 

 a slight incurvation or sinus, a trifle in front of the centre. The posterior end is faintly 

 arcuate, oblique, and joins the upper and lower margins -with rounded angles. The 

 hinge-plate is very slender, and furnished with two unequal teeth in each valve, of which 

 the anterior ones are larger than the posterior, that in the left valve being shaped like an 

 inverted letter V, thus A. Behind the teeth there is a distinct elongate groove for the 

 reception of the internal ligament. The external ligament is very long, extending from 

 n front of the umbones nearly the whole length of the dorsal margin, to which it is 

 attached. The interior of the valves is white and a little glossy. The anterior muscular 

 impression is rather deep, and defined on the posterior side by a straightish thickened 

 edge, and above this there are indications of a second smaller scar. The posterior 

 cicatrix and the pallial line are very indistinct. 



Length 13 mm., height 8, diameter 5^. 



Habitat. — Off Royal Sound, Kerguelen Island, in 25 fathoms. 



