196 THE VOYAGE OF H.M.S. CHALLENGEI!. 



Diplodonta scalpta, n. sp. (PI. XIV. figs. 8-8a). 



Testa subcompressa, alba, subpellucida, insequilateralis, concentrice exillissime striata, 

 lineis elevatis subdistantibus opacis coneinne ornata. Umbones parvi, parum promin- 

 eutes, in f longitudinis positi, paulo prominuli. Mai'go dorsi utrinque rectiusculus, 

 antice intus anguste prelougateque sulcatus, oblique desceudeus, postice magis 

 horizontalis. Latus posticum altum, late arcuatum, superne indistincte subangulatum, 

 anticum acutius rotundatum. Ventris margo late curvatus. Cardo angustus. Dens 

 duplex profunde fissus, intus projectus. Ligamenti sulcus parvus, angustus, supra 

 marginem situs. 



This species has a rather compressed shell. It is thin, semitransparent white, finely 

 striated with the lines of growth, and ornamented at intervals with thread-like white 

 ridges or lirse which, under a lens of low power, have the appearance of concentric scratches. 

 It is somewhat inequilateral, and a little longer than high. The hinder dorsal outline is 

 somewhat oblique and almost rectilinear, the anterior being equally straight, but more 

 sloping. The lower margin is broadly arcuate, and ascends rather more anteriorly than 

 behind. The posterior end is broader or higher, and more gently curved than the front, 

 and at the upper part forms an indistinct obtuse angle with the dorsal margin. The 

 umbones are very small, acute, a trifle antemedian, and only a little elevated above the 

 hinge-plate. This is narrow, and but slightly developed on either side of the cardinal 

 teeth. Of these the double triangular one is remarkable, is deeply slit at the top and 

 projects within the valve beyond the hinge-plate. The other tooth in both valves is 

 narrow, divergent, and not united at the upper extremity to the larger tooth. The 

 anterior dorsal margin has a shallow groove within in both valves, which is peculiar as it 

 reaches to the middle of the lateral outline. The hinder margin is not grooved, but only 

 has a furrow upon the border for the reception of the ligament, which may be described 

 as subexternal. The muscular impressions are scai'cely observable, and too indistinct for 

 description. 



Length 7 mm., height G^, diameter 3^. 



Habitat. — Station 187, off Cape York, North Australia, in 6 fathoms ; coral mud. 



Its compressed form, the prolonged shallow groove within the front dorsal and 

 lateral margins, and the very deeply- fissured tooth projecting within beyond the hinge- 

 plate, are the most prominent features distinguishing this species, 



Diplodonta corpulenta, n. sp. (PL XIV. figs. 9-9a). 



Testa parva, globosa, tenuis, rotundata, subdiaphana, vix inasquilateralis, concentrice 

 tenuiter striata et subdistanter gracillime sublirata. Umbones minuti, parum prominentes, 

 paulo ante medium coUocati. Margo dorsalis antice curvatus, sul^horizontalis, postice 



