REPORT OX THE LAMELLIBRANCHIATA. 127 



carum ornata. Umbones promincntes, valdc incurvati, paulo ante medium siti. Denies 

 tres cardinales in utraque valva. Margo interior valvarum minute denticulatus. 



The form of this shell is longer than high, and somewhat roundly trigonal. The 

 anterior slope descends more suddenly than the posterior, and is arcuate in the lunular 

 region, forming with the upcurvcd lower edge a sharply rounded end. The posterior 

 slope is well arched, and rounds into the ventral margin more suddenly but lower down 

 than the anterior. The exterior surface, with the exception of the hinder dorsal area, 

 is strongly cancellated with radiating and concentric ridges, the former being thicker 

 than the latter. There are about twenty of them in the earlier growth of the shell, 

 exclusive of those on the luuule. Of these a few down th(> posterior side are rather finer 

 and more approximated than the rest, which are broader than the interstices between 

 them. In addition to these twenty, as the ventral margin is approached, other finer inter- 

 calary costse are met with. One of these generally falls in each interstice between the 

 larger ones, and gradually tapering from the edge upwards disappears about the middle of 

 the valve. The concentric ridges are moderately thin behind, thicker and subnodulous 

 on crossing the anterior and central radiating costse, vary from about eight to ten in 

 number in the numerous valves under examination, and are generally fairly equidistant. 

 They are excessively slender at the sides, becoming almost obsolete upon the lunula and 

 the area. The hinge-teeth are three in each A'alve. In the right the anterior is 

 smallest, and not quite parallel with the lunular margin, the two others being subequal 

 and diverging, the hindermost being somewhat bipartite. In the left valve the hind- 

 most is the smallest, very slender, and bounds the ligament. The central tooth is the 

 largest, bifid, situated just under the apex of the umbo, and inclines posteriorly. The 

 front one is more slender, diverging, and directed anteriorly. The beaks are smooth, 

 glossy, hyaline, well incurved, and have a forward direction. Just within the posterior 

 dorsal margin of the right valve there is a shallow groove for the reception of a corre- 

 sponding small ridge in the opposite valve. The denticulation of the margin, commenc- 

 ing at the umbo in front, extends all round, except for a short distance along the 

 posterior dorsal edge, and is a trifle finer near the hinder extremity than elsewhere. 

 Pallial sinus short, angular. 



Length 4^ mm., height 3|, diameter 2|. A variety from Amboina with more 

 slender radiating costellse has a length of 3^ mm., is 3 high and 2^ in diameter. 



Habitat. — Station 188, south of New Guinea, depth 28 fathoms, green mud ; variety, 

 Amboina, 15 to 20 fathoms. 



The cancellation of the surface of this minute species is very coarse for the small 

 size of the sheU, and much stronger than that which obtains in the earlier stages of 

 growth of some of the well-known larger species from the same and neighbour- 



ing seas. 



