132 ALASKA GEOLOGY 



agree with Terebella, a living genus whose oldest known 

 species occurs in the Liassic. The relationship being on 

 the one hand to an Ordovician type and on the other to a 

 Jurassic and living genus, we infer that the period of its 

 existence must have been at some intermediate time; and 

 since the latter relationship is doubtless the more intimate 

 it is, especially in view of the other evidence, justifiable to 

 assume that this period was post-Paleozoic. 



The pelecypod has been called Inoceramya concentrica. 

 The generic name is intended to suggest the supposed 

 relationship of the new type, the general expression of the 

 shell being very like that of a large Posidonomya, a well- 

 marked late Paleozoic and early Mesozoic genus, and from 

 which we believe it originated, while the vertical liga- 

 mentary pits of the hinge plate ally it to Inoceramus, a 

 highly characteristic Cretaceous genus. It is just such a 

 form as might be expected to have given rise to the last 

 genus; while, on the other hand, its derivation from Posi- 

 donomya is scarcely to be questioned. Assuming that 

 Inoceramya is really a connecting link between Posi- 

 donomya and Inoceramus, it is fair to assume further that 

 it existed some time near the extinction of the earlier of 

 those genera and before the later one attained its typical 

 characteristics; i. e., about early Jurassic or Liassic time. 



After weighing, as we have, the evidence of all its 

 known fossils, no other decision seems justifiable than that 

 the slate of the Yakutat series is of Liassic age. 



DESCRIPTIONS OF SPECIES 



VERMES 



Suborder TUBICOLA 



Genus Terebellina gen. nov. 



Long, subcylindrical, gently curved and rather thick -walled tubes, 

 acuminate below; surface obscurely striated transversely. Tubes 

 composed of cemented minute siliceous grains. 



