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



Only a single right valve is at present known, and this probably does not represent the 

 maximum size of the species. It is rather compressed, thin, about equilateral, rather 

 squarely truncate behind, rounded in front, and gentlj'' curved below. The small acute 

 umbo being rather prominent, and the dorsal margin in front rather concave, the upper 

 part of the shell has a peaked appearance. The sculpture consists of fine acute _close-set 

 lirjB, which become more elevated and lamellar on the posterior side, the lamellae being 

 somewhat interrupted down the middle by a shallow impression, so that on the posterior 

 dorsal margin and on a slight angle radiating from the beak to the lower hinder extremity 

 they form two separate crests. There is a single cardinal tooth in this right valve, and 

 a small and remote lateral on each side. The lio;ament lies in a narrow a;roove under the 

 dorsal ed2;e. 



Length 4| mm., height 4j, supposed diameter of the perfect shell 2. 



Habitat. — Station 185b, east of Cape York, North Australia, in 155 fathoms; coral 

 sand. 



Lucina [Divaricella) irpex, n. sp. (PI. XIIT. figs. 4-4a). 



Testa subcircularis, fere sequilateralis, modice convexa, alba, liris elevatis tenuibus 

 paulo ante medium oblique divaricatis instructa, crassiuscula, spatiis incrementi 

 coucentrice gradata. Umbones parvi, vix ante medium siti, antrorsum versi, parum 

 elevati. Margo dorsi postice oblique arcuatus, antice minus curvatus, aliquanto 

 obliquior. Lunula minuta, profunda. Dens cardiualis in valva dextra unicus, crassus, 

 obtusus ; dentes duo valvae sinistras parvi, fossa triangulari sejuncti, posteriori valde 

 obliquo. Fossa ligamenti elongata, utrinque attenuata, intra marginem sita. Cicatrix 

 antica angusta, elongata. Margo valvarum fortiter serratus. 



This species is almost circular, about as long as high, white, almost equilateral, 

 moderately convex, and ornamented with fine, much elevated ridges or costellfB which 

 divaricate at an angle of about 45° at a slightly arcuate and oblique line radiating 

 from the umbo and markino- off from the anterior end rather more than a third of the 

 valve. The ridges are interrupted at intervals which mark periods of growth, by which 

 the surface of the valves present a graduated appearance. The grooves, which are equal 

 to or a little broader than the costella3 between them, are conspicuously deej), especially at 

 the sides, so that the outline is prettily and strongly dentate. The margin is well curved 

 all rouud, except along the anterior dorsal slope, where it is a trifle less arcuate. The 

 beaks are small, curve over towards the front, are but very slightly raised above the 

 hinge-line, located scarcely in advance of the centre, and have a most minute, deejJy 

 sunken lunule immediately beneath them. The hinge is composed of a single thick, 

 obtuse tooth in the rioht valve which fits in between two smaller ones in the left. The 

 ligament is internal, located in an elongate deepish groove, attenuated at both ends. 



