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



Like several other species of Diplodonta, this is mainly distinguished by its form. It 

 is somewhat square behind, and the minute subpunctation of the surface, which is chiefly 

 noticeable towards the hinder part, is a feature of some importance. The hinge and 

 interior are of the usual character. 



Diplodonta sp. 



Habitat. — Port Jackson, in 2 to 10 fathoms. 



One right valve only was obtained. It is of an ordinary rounded form, only 

 moderately convex, as long as high, and presents the usual kind of concentric striation 

 and the normal dentition of hinge. It seems to be distinct from any of the species 

 previously recorded from the same district. 



Diplodonta sp. 



Habitat. — Station 135, Nightingale Island, Tristan da Cunha Islands, Mid South 

 Atlantic, in 100 to 150 fathoms. 



Only a single left valve of this species was obtained. It is old and thickened, and 

 very closely resembles the British Diplodonta rotundata, but is rather more obtuse at the 

 anterior end. The limits of the species in this genus are very difficult to decide, as the 

 main character is that of form, whicli is a very variable one. In sculpture many of the 

 species are alike, and there is hardly any departure from white in the colouring, nor does 

 the dentition materially difi'er in many of the forms. 



Diplodonta apicalis, Philippi. 



Diplodonta apicalis, Philippi, Enum. Moll. Sicil., voL i. jj. 31, pi. iv. fig. 6, vol. ii. p. 24. 

 Habitat.— Teneriie, in 70 fathoms. 



This species has already been recorded from the Canary Islands by the late 

 E. MacAndrew. It is considered by some authors identical with Diplodonta 

 trigonula, Bronn, but Philippi held them distinct. Not having had an opportunity of 

 consulting Bronn's description and figure, or of examining a specimen, I prefer to adopt 

 the name given by Philippi. 



