104 COLLECTIONS FROM MELANESIA. 



in form, the more usual outline being represented by Reeve's fig. 

 47 b, which lie wrongly ascribes to the West-Indian species. The 

 difference in the angle of divarication of the ridges is at once seen 

 by comparing the two figures 47a and 47 b. 



43. Diplodonta subcrassa. (Plate VII. figs. 1, 1 1.) 



Shell roundly subquadrangular, only moderately inflated, thickish, 

 decidedly inequilateral, dirty white, not glossy. Anterior side 

 shorter than the posterior, somewhat pointed at the extremity ; 

 hinder side very broad, squarish, yet rounded. Posterior dorsal 

 margin si raightish, only a trifle oblique; anterior a little concave 

 near the beaks, then feebly curved and suddenly descending. 

 Ventral outline arcuate, considerably upcurving in front. Surface 

 concentrically finely striated, somewhat uneven through periodic 

 interruptions of growth. Umbones small, contiguous, incurved, 

 pellucid, located in advance of the median line. Hinge-teeth 

 rather strong, posterior of the right valve and anterior in the 

 left stout, snbequal, bifid, and the anterior in the fonner is rather 

 thicker than the hinder in the latter, which is sublamellar. Muscular 

 scars somewhat small, deepish, subequal. Interior of the valves 

 not very glossy, roughish. 



Length 15 millim., height 14, diameter 8|. 



J lab. West Island. Prince of Wales Channel, Torres Straits, 7 fms. 



This is rather a solid species and peculiarly acuminated at the 

 extremity in front, with rather conspicuous bines indicating periodic 

 growth. 



44. Diplodonta snblateralis. (Plate VII. fig. K.) 



Shell very inequilateral, much broader behind than in front, 

 scarcely longer than high, moderately inflated, white, finely concen- 

 trically sculptured with the lines of growth. Posterior dorsal 

 margin nearly horizontal, straight at first, then gradually curving 

 into the broadly arcuate lateral outline. Front dorsal edge some- 

 what oblique, scarcely recurved near the beak, but afterwards 

 rounding into the side margin, which is very much more suddenly 

 curvate than at the opposite end. Ventral margin more sharply 

 arched than the posterior, but less so than the anterior. Umbones 

 small, acute, only slightly elevated, situated far in advance of the 

 centre. Hinge-plate narrow in the right valve, conspicuously 

 grooved in front of the two cardinal teeth ; of these the anterior is 

 very small, lamellar, and falls perpendicularly beneath the beak, 

 the posterior being about three times as thick and deeply bisected. 

 The ligamental furrow is narrow and 5| millim. in length. The 

 muscular scars are subequal, the posterior a trifle broader. Length 

 19| millim., height 18^-, diameter 10. 



Nab. Torres Straits, 7-10 fms. 



Only a right valve of this species was obtained by Dr. Coppinger, 

 which, however, is apparently distinct in form, the chief distin- 



