221 



shell clearly belongs to the Anatinidce, while Meekia is supposed to be 

 allied to Trigonia or Tancredia. 



THRACIA SEMIPLANATA. (N. Sp.) 

 Plate 29, figs. 5, 5, 56 and 5c. 



Thracia, Sp. Undt. This volume, page 57. 



Shell transversely elongated, compressed, inequivalve, the right 

 valve being moderately convex and the left almost flat ; nearly equi- 

 lateral : anterior end subangular or obtusely pointed below the middle 

 in some specimens, but rounded in others : posterior end about equal 

 in length to the anterior but squared and truncated or subtruncated 

 almost at a right angle to the ventral margin. Superior border very 

 oblique anteriorly, sloping convexly and rapidly downwards and 

 meeting the upward curve of the basal margin at an obtuse angle ; 

 behind the beaks the superior border descends also obliquely, but not 

 quite so rapidly, and in some specimens very gently and in a straight 

 line or shallowly concave curve, to its junction with the posterior 

 margin : basal margin nearly straight in the centre. Umbones broad, 

 prominent, and nearly central, beaks slightly depressed, curved 

 strongly inwards and backwards. In well preserved specimens the 

 posterior area is bounded by a sharp, but not very prominent ridge or 

 keel, which extends from the beaks to the posterior end of the base, 

 and behind and above which the valves are obliquely inflected. On 

 the hinge line also, behind the beaks, there is a long and narrow, 

 linear-lanceolate and shallowly excavated ligamental area. 



Sculpture somewhat variable ; in some specimens the surface is 

 very finely striated, in others the concentric striations are coarser and 

 consist of closely disposed raised lines which are most densely crowded 

 on the posterior area. 



Length of a large specimen, fifty-six millimetres ; height, thirty- 

 eight mm. ; thickness through the closed valves, twenty mm. 



Abundant and in a riue state of preservation on the South side of 

 Alliford Bay; Bear Skin Bay, Skidegate Inlet, two specimens; 

 South Island, one specimen. 



The above description is intended to express the characters presen- 

 ted by normal and undistorted specimens. In this condition even the 

 species is very variable in form as well as in sculpture. Thus, in some 

 specimens the shell is more or less pointed, though obtusely so, below 

 the middle or near the base anteriorly, in others the anterior margin 

 is rounded. The upper or cardinal margin of the posterior end is 



