Tellina.\ PELECYPODA. 951 



Subgen. 2. ARCOPAGIA (Leach), Brown, 1827. 



Arco/Mf/ia (Leach), in Brown, 111. Conch. Gt. Brit., 1827, p. ii, pi. 10, f. 8. 

 Cydippe, Leach, 1852. TV pe : Tell inn crassa, Pennant. 



Shell large, solid, rounded, moderately convex, the flexure obso- 

 lete ; posterior left lateral absent and the anterior obsolete, other 

 teeth normal ; sinus free, ascending obliquely ; internal radii thick 

 and strong but ill-defined ; sculpture concentric, usually smoothish 

 or not sharply lamellate, sometimes reduced to incremental lines. 



Warm, temperate, and tropical seas. 



The chief feature of this group is the free sinus, but this in species 

 otherwise closely allied becomes more or less confluent. (Dall.) 



6. Tellina disculus, Deshayes, 1855. Plate 59, fig. 13. 



Tellina disculus, Desh., P.Z.S., 1854 (1855), 360 ; Conch. Icon., xvii, 1866, 

 f. 306 ; Romer, Conch. Cab., x, 1871, 79, pi. 22, f. 4-6 ; Crit. List., 42 ; 

 M.N.Z.M., 143 ; Hutton, P.L.S. N.S.W., ix, 521. T. lactea, Q. & G. : 

 Gray, Dieff. N.Z., 254 ; not of Quoy and Gaimard. T. sublenticularis, 

 Sowerby : Hutton, C.M.M., 67 ; J. de Conch., xxvi, 47 ; not of Sowerby. 



Shell suborbicular, rather thick, strongly concentrically striated, 

 yellowish-white, the right valve more convex than the left. Beaks 

 not much produced, situate a little behind the middle, slightly in- 

 clined towards the ligament. Anterior end regularly rounded, semi- 

 circular, the dorsal margin descending, nearly straight ; posterior 

 end shorter, very obtusely subangular, with a shallow groove on the 

 left valve and round ridge on the right, the dorsal slope straight, 

 descending ; basal margin broadly rounded. Sculpture consisting of 

 strong, sharp, equidistant and close concentric stria3, with traces of 

 radiate lines. Colour dirty - white, yellow at the beaks. Interior 

 yellowish in the middle and umbonal cavity, white at the margins, 

 smooth, shining, the radii not well pronounced. Hinge : Right valve 

 with 2 divergent cardinals, the posterior stronger, 1 triangular an- 

 terior lateral tooth a short distance in front of the cardinal and a more 

 distant triangular posterior lateral ; left valve with 2 not very high 

 cardinals, the anterior much stouter, the posterior an oblique thin 

 lamella, the lateral teeth obsolete. Ligament elongate, strong, raised. 

 Adductor-scars subequal, high and oval, the anterior a little larger. 

 Pallial sinus large, triangular, reaching in front to within a short 

 distance of the muscle-scar, the lower part almost confluent with the 

 pallial line. 



Length, 38 mm. ; height, 38 mm. (type). Length, 37 mm. ; 

 height, 33 mm. 



Type in the British Museum. 



Hob. North Island ; Banks Peninsula ; Chatham Islands. 



Fossil in the Pliocene. 



