159 



short, its margin broadly rounded below the middle, straighter above ; 

 posterior side short, obh'quely truncated, and apparently angular or sub- 

 angular at its Junction with the ventral border below ; basal margin regu- 

 larly rounded in front, probably straighter behind. Umbones oblique, 

 enormously developed ; beaks situated in advance of the middle, acute, 

 semispiral, an^l divergent at their apices. Behind the beaks a blunt but 

 well defined keel or ridge extends to the base of the posterior end and 

 separates a wide posterior area from the main bod}^ of the shell. This 

 posterior umbonal keel is curved convexly above, but becomes much 

 straighter below, and the posterior area, which is equal in height to the 

 shell itself, is tlattened at almost a right angle to the central portion of 

 the valves. Lunule distinctlj' defined, ovate cordate in outline, not 

 quite half the height of the shell, a little higher than broad, flatly ex- 

 cavated at uearlj' a right angle to the middle jjart of the ralves. 



Sui'face marked wJth concentric stria; of growth. 



Hinge plate of the right valve (the only one known) very large, 

 broadly triangular, and bearing in its centre a transversely elongated 

 tooth, which is narrow and prominent above, but dilated and obliquely 

 truncated below. On the anterior or lunular side this tooth is united to 

 the hinge plate above, but on the posterior side it is separated from it 

 by an oblique and probably ligamentary groove which runs parallel to 

 the fulcrum. On either side of the tooth there is a deep, ti*ansversely 

 elongated pit, evidently for the reception of two teeth in the left valve. 



S. \Y. side of Denman Island, in Division B ; J. Richardson, 1871. An 

 imperfect right valve, with the surface much worn, and partly covered 

 with the base of attachment of a living species of Bahnus. 



The lower half of the posterior margin of this specimen is badly 

 broken, so that the exact outline of this part of the shell has yet to be 

 ascertained. The lines of growth and the direction of the post-umbonal 

 groove seem to indicate that the posterior margin formed a sub-angular 

 junction with the ventral border, but it cannot be positively asserted 

 ehat the base was not rounded behind. Although the characters of 0. 

 Vancouver ends are rather imperfectly known, yet they appear to be suf- 

 ficiently intelligible to pre\ ent it being confounded with any previously 

 described species. In the general shape of the shell, and especially in 

 the contour and proportionate size of the lunule and posterior area, there 

 are many points of resemblance between it and Opisoma Geinitziana of 

 Stoliczka,* but the present species is a true Opts, and has one tooth in the 



• Cretaceous Pelecypoda of Southern India, p. 2SS, pi. 10, flg, 11. 



