Pholadidea.} PELECYPODA. 1015 



valves prolonged behind into leathery and testaceous cups or a 

 tube (siphonoplax). 



Distribution. European seas, west coast of America, New Zealand. 



Fossil in the Tertiary. 



KEY TO SPECIES. 



A. Siphonoplax horny, without inner calcareous plates . . . . spathulata. 



B. Siphonoplax horny, with an inner tridcntate calcareous plate on 



each side . . . . . . . . . . . . tridens. 



1. Pholadidea spathulata, Howerby, 1850. Plate 61, fig. 14. 



Pholas spathulata, Sow., P.Z.S., 1849 (1850), 1(52 ; Conch. Icon., xviii, 1872, 

 f. 45. Pholadidea spathulata, Sow. : Hutton, J. de Conch., xxvi, 43 ; 

 M.N.Z.M., 133. 



Shell elongated, inflated, very inequilateral, closed in front in the 

 adult, with a horny posterior cup with spathulate sides, anterior 

 upper side decussate, posterior part concentrically ribbed, white, 

 thin and fragile. Beaks not visible from the outside, incurved, blunt. 

 Anterior end short, globose, angularly rounded in front, the dorsal 

 margin convex, descending ; posterior end long, acuminate, the shell- 

 margin truncate, dorsal margin straight. The siphonoplax brown, 

 horny, compressed, margined behind, about twice as high as its 

 length, open at the end and below. Protoplax consisting of 2 equal 

 calcareous laminae, united in the middle by a membrane, tapering and 

 pointed in front, with a triangular striated posterior lobe. Metaplax 

 consisting of 2 long and narrow, posteriorly sharply pointed cal- 

 careous plates, the left broader, united by a thin horny mem- 

 brane. Sculpture : An oblique groove descending obliquely and slightly 

 backwards from the beaks towards the ventral margins divides each 

 valve into an anterior and posterior part, the latter being somewhat 

 longer ; the anterior lower callous plates are nearly smooth, with 

 fine concentric and radial lines, the dorsal part is concentrically 

 closely ridged and radially striated at the anterior half, the radial 

 ribs raised and forming sharp triangular teeth ; posterior part with 

 somewhat irregular flattish concentric riblets. Epidermis thin, horny, 

 light brown, extending beyond the margins of the valves. Colour 

 white, the siphonoplax dark brown. Interior white, polished, smooth. 

 Margins thin and sharp, smooth. Hinge enclosing the umbones in a 

 half-circle, rounded, without teeth, but in each umbonal cavity a long, 

 thin, descending apophysis. Eesilium small, internal, in front of the 

 beaks. Posterior adductor-scar dorsal, elongated. Pallial sinus broad 

 and deep, broadly rounded in front, extending as far as the oblique 

 groove, which appears as a rib on the inside. 



Length, 45 mm. ; height, 21 mm. ; diameter, 19 mm ; length of 

 the siphonoplax, 7 mm. 



Type in the British Museum. 



Hob. North and South Islands, as far south as \Vaikouaiti. 

 Otago ; boring in soft rock and clay. 



Fossil in the Pliocene. 



