HEPORT ON THE LAMELLIBRANCHIATA. 197 



oliliqiie le\'iterque arcuatus ; ventris margo aliquanto late arcuatus. Latus anticum 

 altuni, obtusum, posticum paulo acutius rotundatum. Dentos cardinis in valva sinistra 

 normales, antico bifido, crasso, postico gracili, elongate. Margo dorsi ante umbonem 

 iutus profunde efFossus, supra dentem bifidum levissime emarginatus vol depressus. 

 Ligamenti sulcus baud profundus, marginalis, angustus. Impressiones musculorum 

 magnag, elongatse. 



The shell of this small species is thin, rather globose, rounded, a little longer than 

 high, concentrically finely striated, and exhibits at short distances minute elevated ridges 

 or lirse. It is white, slightly glossy, and inequilateral, the anterior side being a trifli' 

 shorter than the posterior. The front dorsal mai'gin is very slightly concave near the 

 beaks, then somewhat excurved, rising a little befoi'e commencing the descent to the side. 

 The hinder margin is feebly arcuate, and slopes more obliquely than the anterior. The 

 ventral outline is broadly curved, and forms with the dorsal margin a sharper curve 

 behind than in front. The umbones are very small, only a very little elevated above 

 the hinge-plate, not very acute at the tip, and are situated a trifle in advance of the 

 centre. In the left valve the front tooth is stout, strongly grooved at the top, falls 

 immediately beneath, the tip of the beak, and inclines but very slightly towards the front. 

 The hind tooth is very slender and oblique, elongate, and is almost connected with the 

 anterior tooth at the upper end, and at the lower extremity runs into the inner edge of 

 the hinge-plate, being separated from the other tooth by a deep triangular pit. The 

 anterior half of the hinge-plate is just a trifle broader than the posterior, remarkablj' 

 deeply and widely grooved. The hinder portion is also deeply channelled, a narrow 

 elongate slip behind the umbo being partitioned ofi" for the ligament, which would 

 scarcely be visible externally. The muscular scars are elongate and very large. 



Length 4^ mm., height 4^^, diameter 3. 



Habitat. — Station 187, west of Cape York, North Australia, in 6 fathoms; also 

 Station 188, a little farther west of Cape York, at a depth of 28 fathoms. 



This species is distinguished from Diplodonta suhglobosa by its more ventricose form. 

 The anterior end is higher and more obtuse than the posterior, whilst in the other species, 

 on the contrary, the latter extremity is the more broadly arcuate. The hinge-plate, too, 

 in the present species is broader and more deeply grooved, the posterior tooth in the 

 left valve is longer and more oblique, the ligamental furrow is narrower, and the double 

 tooth is not so deeply bipartite as in the following species. 



Diplodonta suhglobosa, n. sp. (PI. XIV. figs. 10-lOa). 



Testa parva, rotundata, subglobosa, tenuis, baud nitida, striis concentricis tenuis- 

 simis sculpta, semipellucido-alba, paululum insequilateralis. Umbones parvi, parum 



