224 



PLEUBOMYA (SUBCOMPRESSA ? VAR.) L^EVIGATA. 

 Plate 30, figs. 1, la, 11>, and Ic. 



Myadtes subcompressus, White. 1880. United States Geological Sur- 



vey, Contributions to Palaeontology, 

 Nos. 2-8, p. 151, pi. 38, figs. 56 and 

 5c : ccet excl. 



Shell compressed, most convex near the anterior margin and some- 

 what wedge-shaped as seen from above. Valves closed in front, 

 slightly gaping behind, transveresly elongated, the length being twice 

 the height in some specimens and less than twice in others, very inequi- 

 lateral : anterior end short and truncated or sub-truncated at almost a 

 right angle to the ventral margin : posterior end much longer and 

 either rounded or sub-truncated somewhat obliquely at its extremity. 

 Cardinal margin short, excavated and sloping suddenly downwards in 

 front : straighter and descending much more gradually behind : ventral 

 margin nearly straight. Umbones large, broad and prominent : beaks 

 small, curved inwards, downwards and a little forwards, and either 

 anterior and nearly but not quite terminal or placed about half way 

 between the centre of the superior border and its anterior termination. 

 Anterior umbonal ridge obtusely angular and extending from the beaks 

 to the anterior end of the basal margin : in front of this ridge or pro- 

 minence the valves are bent obliquely and abruptly inwards, and 

 immediately behind it there is a broad and faint shallow depression : 

 posterior umbonal ridge well defined above, but becoming gradually 

 obsolete below. 



Surface marked with fine and rather crowded concentric striations. 

 Hinge teeth and muscular impressions unknown. 



In two specimens the maximum height is twenty millimetres and 

 the thickness twelve, but the length of the one is forty mm., while that 

 of the other is only thirty-three. 



South side of Alliford Bay, nine casts of both valves : East end of 

 Maud Island, three similar casts. This form occurs also in the felsites 

 of the Iltasyouco Eiver, B.C., where it was collected by Dr. G. M. Dawson 

 in 1876. The specimens from this latter locality were doubtfully and as 

 it would now seem erroneously identified with the Pleuromya subellip- 

 tica of Meek and Hayden in the Eeport of Progress of the Geological 

 Survey of Canada for 1876-77. 



The shells described above appear to be precisely similar to two 

 Montana specimens which are figured by Dr. C. A. White as varieties 

 of Myadtes subcompressus, Meek, although on the otherhand it is diffi- 

 cult to see how they can be distinguished from some of the Gault 



