71 



this end, between each of the two rows. The rest of the rows curve 

 obliquely from the posterior area to the ventral border. One specimen 

 shews ver}' coarse and concentric raised strijx? of growth, with correspond- 

 ing grooves ; also that the tubercles are longitudinall}' elongated. It is 

 possible, indeed, that instead of rows of distinct and sejxu-ate tubercles, 

 the true sculpture may have consisted of oblique ribs which are inter- 

 rupted and made tubercular bj' their being crossed by the concentric 

 grooves just described. 



The posterior area is transversely striated, and in one individual there 

 is a faint longitudinal groove, bordered by an obtuse ridge, which runs 

 the gi'eater part of the length ot its outer margin. Occasionally there 

 are a few scattered tubercles in this region, but these are often absent, 

 and when present are generallj-, though not always, most numerous near 

 the beaks. 



As none of the specimens are perfect or undistorted, it is not possible 

 to give sufficiently exact measurements. 



In 1873 Mr. Eichardson collected a solitary specimen of a Trigonia at 

 North West Bay, Vancouver Island, in a very fair state of preservation, 

 which, in the Avriter's judgment, is scarcely more than a variety of Tri- 

 gonia Tryoniana of Gabb.* Judging by the examples described above, 

 the difterences between the Yancouver and Queen Charlotte Island shells 

 seem slight, but the resemblance may be more apparent than real, and 

 may be mainly due to a want of knowledge of the distinctive sculpture 

 of the latter. 



The general shape, and the surface markings on the posterior area, are 

 essentially the same in both. In the Vancouver fossil the central part of 

 the valves is traversed by fifteen or sixteen oblique ribs, which commence 

 at the outer limit of the p)osterior area. Xine of these terminate at the 

 anterior margin and six cross to the ventral border, and the ribs increase 

 more and more in their distance from each other towai'ds the posterior 

 extremit}'. The costa^ are crossed by concentric and coarse grooves 

 which give a subtubercular aspect to the ribs ; in the umbonal region the 

 tubercles are entirely distinct and separate. The upward bend of the 

 tubercles just before reaching the anterior margin, and the occurrence of 

 a single isolated one alternating at that end with each continuous series, 

 two features which seem to be characteristic of the Queen Charlotte 

 Island specimens, cannot be traced in the Trigonia from Vancouver. 

 These are the only differences of any consequence that can be observed 



* "Paloeontulogyof California,'' Vol. I, pp. 184-9. Plate XXV., fig-. 176. 



