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. 
