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



Only a single minute valve of this species is at present known, which probably does 

 not nearly approach the full size it attains in a more adult condition. Considering the 

 distinct character of the sculpture, and that it is very different from all the known species, 

 I have thought it advisable to describe this species although represented by such slight 

 material. 



Davila, Gray. 



Davila (?*) umbonata, n. sp. (PI. VI. figs. 1-1^). 



Testa transversim ovalis, mediocriter insequilateralis, albida, polita, incrementi lineis 

 concentricis irregularibus sculpta. Umbones alti, product!, ad apicem erosi. Latus 

 anticum breve, acute rotundatum, posticum longius, latius curvatum. Margo dorsahs 

 anterior levissime arcuatus, valcle obliquus, posterior minus descendens, subrectilinearis, 

 ventralis late arcuatus. Dentes cardinales tres in utraque valva, cjuorum duo ante liga- 

 meutum siti sunt. In valva dextra anterior gracilis, prope marginem locatus, in sinistra 

 medianus minimus. Impressioues musculares irregulares, antica minore. Linea pallii 

 postice vix siuuata. Cartilage interna magna, fusca. Ligamentum aliud linearis infra 

 umbones situm. 



The shell is thinnish, white, glossy, and striated by fine lines of growth, at intervals 

 some of the strise being deeper than the rest, as if marking off periods of growth. In 

 front a large lunule is marked off by a faint line not seen by the naked eye. The form is 

 nearly transversely oval, except that the dorsal outline is interrupted by the beaks, which 

 are situated a little excentrically and rather nearer the anterior side. They are eroded at 

 the tips, a trifle prominent alcove the hinge-line, and scarcely turned towards the front. 

 The hinge is composed of three unequal and divergent teeth in each valve, a largish 

 internal cartilage having two of the teeth in front and the third posterior to it, and in 

 addition a linear ligament along the dorsal edge immediately under and on both sides of 

 the tips of the umbones. In the right valve the foremost tooth is slender, close to and 

 parallel with the outer edge of the valve. The second or median tooth is also slender, 

 a trifle larger than the anterior, from which it diverges somewhat, yet having a forward 

 direction. The hindmost, likewise narrow, inclines backwards, and is separated from the 

 outer edge by a deepish groove. In the left valve the anterior tooth is decidedly the 

 largest, separated from the margin with which it is subparallel by a deepish furrow. 

 The posterior is somewhat smaller, prominent, and close to the outer edges, and the 

 median one is very small, just under the umbo, and scarcely rises to the level of the 

 dorsal margin. The interior of the valves is glossy and white. The muscular impressions 

 are subequal in size and rather irregular in outline. The pallial line is rather remote 

 from the ventral margin, and exhibits the slightest indication of a j)osterior sinus or 

 emargination. 



