198 THE VOYAGE OF H.M.S. CHALLENGIER. 



prominuli, aliquanto ante medium siti. Margo dorsi anticus leviter obliquus, subarcuatus, 

 posticus minus descendens, curvatus. Latus anticum postico angustius, arcuatum. 

 Margo ventris late curvatus, utrinque subsequaliter adscendens. Dentes cardinis inaequales 

 divergentes, antico valvse sinistra;, et postico in dextra crassis, insigniter bifidis. Margo 

 anticus intus sulco elongato baud profundo exaratus. Fossa ligamenti elongata, intra 

 marginem coUocata. 



This species does not probably attain large dimensions. It is a little longer than 

 high, rounded, slightly inequilateral, the posterior side being a trifle the longer. It is 

 thin, semitransparent, not glossy, colourless and finely concentrically striated, the striae, 

 however, under a compound microscope appearing rugose or even granulose. The 

 dorsal outline is curved on both sides, and does not slope much. The obliquity is a trifle 

 greater anteriorly than behind, where the arcuation is a little more pronounced than in 

 front. The posterior end is certainly higher than the anterior, not much curved, and 

 shouldered or obtusely and indistinctly angled where it runs into the dorsal margin. The 

 lower outline is broadly arcuate, and ascends almost equally at each extremity. The 

 umbones are small, a trifle antemedian, and only a little elevated above the niargin. In 

 the right valve the hinder tooth and in the left the anterior are twice as stout as the 

 others and very deeply bifid, the former sloj)ing towards the posterior end and the latter 

 towards the front. The anterior dorsal margin in both valves is grooved within, the 

 furrow being rather long and shallow, and commencing at the front tooth extends about 

 half-way to the middle of the anterior end. In the right valve the dorsal outline, just 

 over the front slender tooth, is faintlj^ emarginate or feebly notched, or, in other words, 

 slightly pressed over towards the tooth. The groove for the ligament is within the outer 

 edge, elongate, rather broad and deep, so that little, if any, of the ligament would be 

 visible exteriorly. The muscular scars are indistinct, but appear to be elongate and 

 moderately large. 



Length 4|^ mm., height 4, diameter 2f. 



Habitat. — Station 187, near Cape York, North Australia, at a depth of 6 fathoms; 

 also Flinders Passage, in 7 fathoms. 



Diplodonta consjncua, n. sp. (PI. XIV. figs. 11-lla). 



Testa trigono-rotundata, alba, tenuis, subsequilateralis, concentrice exilissime striata. 

 Margo dorsi antice arcuatus, j^ostice vix longior, sequaliter curvatus, utrinque subsequaliter 

 obliquus. Margo ventris late arcuatus, ad utrumque latus paulo adscendens, in 

 dorsalem acute-rotundatim ambiens. Umbones parvi, acuti, aliquanto ante medium siti, 

 parum prominuli. Dens anticus valvae dextrse parvus, obliquus, posticus crassus, 

 triangularis, conspicue in medio fissus, in valva sinistra parvus, gi'acUis, antico magno 



