164 CONTRIBUTIONS TO CANADIAN PALAEONTOLOGY. 



advance of the midlength ; front margin subtruncated in the centre ; 

 posterior extremity bluntly jwinted. 



Ventral valve rather more convex than the dorsal; the umbo of the 

 former moderately prominent, its beak incurved and slightly decurved 

 as well as somewhat obliquely truncated ; deltidium very small as the 

 central portion of the anterior margin of the truncated beak of the 

 ventral valve almost touches the beak of the dorsal ; foramen rather 

 large and apparently complete. In an end view the front margin of 

 the ventral valve is seen to be sinuated, the sinus being very shallow 

 and indistinctly defined, but moderately broad and nearly straight in 

 the centre. At a short distance from the front margin the mesial 

 fold and sinus become obsolete. 



Dorsal valve smaller as well as less convex than the ventral, its beak 

 small and depressed, and its front margin provided with a shallow me- 

 sial fold which fits into the corresponding sinus of the opposite valve. 

 The foregoing description is applicable only to the adult shell. In 

 immature specimens the marginal outline is very nearly circular, the 

 length and breadth are almost equal, and the shallow fold and sinus of 

 the front margin of the valves are not developed. 



The surface markings ai-e very imperfectly preserved, but the exte- 

 rior appears to have been neaily smooth, and marked only by concen 

 trie lines of growth and by minute and crowded radiating stria:;. 



The markings on the interior of the valves are also very imperfect- 

 ly preserved. In a cast of a dorsal valve there is a longitudinal medi- 

 an groove less than half the entire length, which probably represents 

 the septum. In the same cast the posterior adductor appears to have 

 been long, narrow, pointed at both ends, and more convex on its inner 

 margin than externally, while the anterior adductor, though also long, 

 narrow and pointed postei-iorly, is narrowly rounded and somewhat 

 dilated in front. 



Dimensions of an average adult specimen : maximum length, seven- 

 ty-one millimetres; greatest breadth, forty-eight mm. and a half; 

 thickness through the closed valves, forty-two mm. 



Seventen more or less perfect casts of the interior of the closed 

 valves were collected, some of which have portions of the test adher- 

 ent thereto. 



Two specimens of Ji Terehratula collected by Mr. James Eichardson 

 in 1872 from the "Lower Shales and Sandstones" of Skidegate Inlet 

 in the Queen Charlotte Islands, appear to be immature individuals of 

 this species. 



