22 



length ; anterior dorsal slope short, almost straight, passing gradually into rounded anterior end ; 

 posterior slope very long, somewhat concave ; ventral outline evenly curved to about middle, 

 whence it curves up to form lower slightly concave margin of rostrum ; sculpture consisting of 

 very minute concentric striae over entire surface of valves, thèse turn sharply at right angles 

 from the outer keel to the concave area of the escutcheon and two radiating ridges or keels, 

 first running from umbo to upper angle of rostral extremity and second from umbo to lower 

 angle; hinge-plate moderately strong, anterior plate lying short distance inwards from margin, 

 posterior extending about two thirds of posterior dorsal slope -, both plates with numerous close 

 set, triangular teeth ; a low, curved longitudinal internai ridge présent in the rostral région. 

 Shell of a pale straw colour, nacre white. 



Measurements of Holotype : — Length 1 1.2 mm., maximum height 4.5 mm., diameter 3 mm. 



Habitat: — The "Siboga" Expédition obtained a complète shell and a few valves of this 

 interesting species at St. 52 (9°3'.4S., ii9°56'.7E., 959 mètres), St. 178 (2°4o'S., i28°37'.5E., 

 835 mètres) and St. 212 (5°54'.5S., i20°i9'.2E., 462 mètres). 



Remarks: — N. {T.) inodica is allied to N. [T.] ramsayi (Smith) ^) from off Sydney, 

 N. S. Wales. Iredale -) has suggested this species as the type of his new genus Thestyleda^ 

 it is another example of a new genus being proposed without characterization. 



9. Nucidana [Thestyledd) scalata^ sp. nov. (Plate I, ngs. 30, 31). 



N. {T.) scalata is allied to N. {T.) ramsayi (Smith) ^) and N'. [T.) modica described 

 above. From both thèse species, however, it is distinguished by its much larger size, its much 

 thinner shell, the shell sculpture consisting of low, very distantly placed rugae, which become 

 very pronounced and almost like ridges on the rostrum, and its comparatively feebly developed 

 hinge teeth. The ridges on the upper edges of the rostrum are distinctly raised and give the 

 shell a scaly appearance in this région. 



Measurements: — Length 13mm., maximum height 5 mm., diameter 2.5 mm. 



Habitat : — A single spécimen of this beautiful species was brought up in a deep-sea 

 trawl at St. 300 (io°48'.6 S., 123° 23.1 E., 918 mètres). 



10. Niiailana [Thestyleda) elaborata^ sp. nov. (Plate I, figs. 32 — 34). 



The following is a description of this new species : — Shell of médium size, thin, elongate, 

 slightly tumid, acutely rounded anteriorly, with a narrow, elongate beak posteriorly, margin 

 of rostrum almost straight ; very inequilateral ; acute, lightly sculptured umbones lying in anterior 

 third of total length ; anterior dorsal slope short ; posterior slope more than twice as long, very 

 concave in first half, then almost straight ; sculpture consisting of very well marked, regular, 

 concentric rugae, almost like ridges, over entire surface of shell, thèse turn sharply, almost at 

 right angles from the outer keel upwards and are divided by a longitudinal groove externally 

 to the second keel into somewhat deflected ridges ; two radiating ridges forming keels and a 

 longitudinal groove midway between keels ; hinge-plate moderately strong, similar to that of 



1) Smith, E. A. — "Challenger" Lamellibranchia, p. 241, pi. XX, figs. 3, 3a (1885). 



2) Iredale, T. — Rec. Austral. Mus., XVII, p. 158 (1929). 



