1016 PELECYPODA. [EulamettibrancMa. 



2. Pholadidea tridens, Gray, 1843. Plate 62, fig. 14. 



Tftlona tridens, Gray, Dieff. N.Z., 254. Pholas tridens, Gray, Conch. Icon., 

 xviii, 1872, f. 38 : Crit. List, 40. Pholadidea tridens, Gray, C.M.M.,59 ; 

 Ereb. & Ter., 5, pi. 2, f. 8 ; Button, J. cle Conch., xxvi, 43 ; M.N.Z.M., 

 133 ; P.L.S. N.S.W., ix, 512 ; Index, 88. 



Shell always smaller than the last species, pyriform, thin and 

 fragile, very inequilateral, closed in front in the adult, with a promi- 

 nent siphonoplax, which has a marginal rounded rib on the side of 

 the valves, and a median groove on each side. Beaks not raised, 

 not visible on the outside, incurved, acute, directed forwards. 

 Anterior end closed in the adult, globose, dorsal margin convex ; 

 posterior end much longer, acuminate, the shell truncated, dorsal 

 margin straight, slowly descending ; ventral margin straight or 

 slightly concave. The siphonoplax horny or partly calcareous, about 

 as high as long, with a broad, rounded rib on the anterior side, and a 

 deep groove on the middle of the right and left side, inside with a 

 small tridentate shelly piece on each side. Protoplax consisting of 

 2 narrow plates, each with a small posterior lobe ; the plates are 

 united by a thin horny membrane, and united with the valves for 

 the whole length. The metaplax also forms part of the valves ; the 

 lamellae are long, tapering to a sharp point in front, the right lamella 

 slightly broader. Sculpture : The diagonal groove meets the margins 

 at the middle of their length ; most of the anterior part is occupied 

 by the convex callous plates, which are finely concentrically striated, 

 the upper triangular part is transformed into a rasp by concentric 

 and radial riblets ; posterior part with broad, flat, concentric ribs. 

 Epidermis thin, horny, extending beyond the margins of the valves. 

 Colour light brown or dirty-white. Interior white, polished, the outer 

 diagonal groove producing a prominent rib. Margins smooth and 

 sharp. Hinge with the umbones in an oval pit ; each valve with a 

 small, thin, obliquely descending apophysis in the umbonal cavity. 

 Resilimn in the cavity of the protoplax in front of the beaks. 

 Posterior adductor-scar dorsal, impressed, elongately oval. Pallia] 

 sinus broad, rounded in front, extending to the median rib. 



Length, 20mm. ; height, 11 mm. ; diameter, 10mm. 



Type in the British Museum. 



Hob. Northern parts of the North Island, together with the 

 foregoing species, but not so common. Also Ecuador coast. 



Genus 2. BARNEA, Kisso, 1826. 



Jin nn n, Ri.sso, Hist. Eur. Mt-r., iv, 1826, 376. Type: B. spinosn, Risso 

 (= P. Candida. L.). Pholas, Lamarck, 1801 ; not of 1790. Cyrtopleura, 

 Tryon, 1862. Holopliclas, Fischer, 1887. 



Animal with the siphons naked at the base, the orifices cirrated. 



Shell oblong-ovate, valves with the anterior gape always open ; 

 accessory plates not exceeding 2 in number, protoplax and mesoplax ; 

 space below the umbonal reflection not divided into cellular cavities 



