838 PELECYPODA. [Protobranchia, 



ribs, colour greyish-olive. Beaks situate a little behind the anterior 

 third, approximate, slightly directed forward, small and low. Anterior 

 end rounded, the dorsal margin somewhat sloping, straight ; pos- 

 terior end truncated, emarginated, with a short and rounded rostrum 

 above, the dorsal margin lightly concave ; basal margin nearly straight, 

 but faintly sinuate at the posterior end. There is neither a distinct 

 lunule nor an escutcheon. Sculpture consisting of a distinct carina 

 extending from the posterior part of the umbones to the angle formed 

 by the basal and posterior margins, dividing the surface in an anterior 

 and posterior area ; the wiiole surface adorned with rather distant 

 narrow and sharp concentric riblets with broader interstices ; on the 

 posterior area the riblets are sinuous, and there is a shallow depres- 

 sion extending from the posterior sinus to the beak. Epidermis thin, 

 not shining. Colour olive-green, greyish, or brownish-green. Interior 

 bluish-white, porcellanous ; margins sharp, simple. Hinge-line very 

 little angulate, with numerous V-shaped small teeth increasing in 

 size from the beaks distally ; the anterior series shorter than the 

 posterior. Ligament parivincular and opisthodetic, wholly external. 

 Adductor-scars not much impressed, oval, subequal. Pallial line dis- 

 tinct, with a fairly deep broadly rounded sinus. 



Length, 36 mm. ; height, 18 mm. ; diameter, 13-5 mm. (type). 



Type in the Mus. Hist. Nat., Paris. 



Hab. New Zealand, very rare (Q. & G.) ; off Great Barrier Island, 

 in 110 fathoms ; Rangitoto Channel (T. F. Cheeseman) ; Wellington 

 Harbour (A. Haylock) ; Port Pegasus, Stewart Island, in 18 fathoms 

 (Captain Bollons). 



Fossil in the Miocene and Pliocene. 



Genus 3. PLEURODOX, S. Wood, 1840. 



Pleurodon, S. Wood, Charles worth's Mag., iv, 1840, 230. Type : P. ovalis, 

 Wood ; not Pleurodonte, Fischer, 1808. Nuculina, d'Orbigny, 1843. 

 Nucinella, Wood, 1850. Huxleyia, A. Adams, 1860 ; not of Bowerbank. 

 Cyrilla, A. Adams, 18(10. 



rjj'P Shell externally resembling Nucula, but having a structure much 

 less nacreous. It is very small, oval or subtrigonal, oblique, not 

 nacreous, equivalve, smooth, inequilateral ; the anterior side short 

 and truncated, the posterior end elongated ; ventral margin smooth ; 

 hinge-line short anteriorly, cardinal border produced and angulated 

 externally. Cardinal plate broad, with a prominent posterior lateral 

 tooth in the left valve, the tooth being received in a depression of the 

 plate of the opposite valve, the edge often turned up like a tooth ; 

 between the lateral tooth and the fosette there are 3 anterior cardinal 

 teeth, very variable in form and arrangement. Ligament in an ex- 

 cavation under the angle of the cardinal border, nearly or quite covered 

 by the margins of the valves. The anterior muscle-scars smaller than 

 the posterior scars ; pallia! line simple. There are no dorsal areas. 



