REPORT ON THE LAMEELIBRANCHIATA. 120 



rotundata. Margo dorsalis anterior \al(lc declivis, leviter arcuatus, posticus minus 

 obliquus, aliquanto curvatus, ventralis late arcuatus. Lunula mediocris, distincta, 

 incrementi lineis fortibus striata, costa radianti, quam cseteris minori, circumdata. 

 Dentes cardinales tres in vulva dextra, divergentes ; anterior parvus, angustus, antice 

 directus, medianus erectus, crassior, sub apice umbonis situs, posticus maximus, retror- 

 sum iuclinatus. Margo valvarum intus minutissime denticulatus. 



The length of the shell is a trifle greater than the height. It is white, inequilateral, 

 the anterior end being the shorter and somewhat sharply rounded, the hinder extremity, 

 on the contrary, being obliquely subtruncate, terminating in a rounded angle towards the 

 lower part. The dorsal margin is very oblique in front and only feeljly arcuate, pos- 

 teriorly it is almost horizontal, and likewise but little arched. The ventral outline is 

 broadly curved, ascending rather more anteriorly than at the opposite extremity. The 

 exterior surface of the valves is ornamented with strong radiating ribs, which, attenuating 

 from the margin upwards, become obsolete towards the umbones. They are about 

 eighteen in number, rounded, about as broad as the grooves between them, subequal in 

 thickness with the exception of a slender one enclosing the lunule and three or four on 

 the posterior side, rather finer than the rest. The slender one in front and two or three 

 following ones are somewhat tvibercular, being crossed by elevated lines of increase. In 

 the deepish furrows between the costse there are numerous very fine but somewhat 

 distant slightly raised concentric lirse. The lunule is distinct, elongate heart-shaped, 

 marked with rather strong raised lines of growth and a little prominent down the 

 middle. The beaks are small, not much raised above the hino;e-line, incurved and 

 directed towards the front. The cardinal teeth are three in number in the risht and 

 probal)ly so in the left. They are separated and divergent. The central one is erect, 

 triangular, and situated perpendicularly beneath the apex of the umbo. The front one 

 is the narrowest, and inclines anteriorly ; the posterior is a trifle longer than the central, 

 but scarcely so thick, directed backward, and probal:>ly in well-preserved specimens is 

 somewhat bipartite at the top. The crenulation of the margin of the valves extends all 

 round excepting upon that portion occupied by the hinge-ligament. It i.-< exccssivel)^ 

 fine, particularly upon the lunular and posterior edges. The muscular impressions and 

 the mantle-mark are too indistinct for description. 



Length 6 mm., height 5. 



Habitat — Off Levuka, Fiji Islands, at a depth of 12 fathoms. 



This species has a cardium-like aspect, and is remarkable for the inconspicuous 

 character of the concentric sculpture, the strong radiating ridges, and the deep grooves 

 separating them. 



(ZOOL. CHALL. EXP. — PART XXXV. — 1885.) Mm 17 



