YAKUTAT FOSSILS 
135 
Considering the importance of the genus as a probable connecting 
link between the Paleozoic and early Mesozoic Posidonomya and the 
chiefly Middle and Upper Cretaceous genus Inoceramus , it is unfor¬ 
tunate that the material upon which the new genus is founded is not 
more complete and better preserved. Still, by careful preparation it 
has been made to show sufficient characters to give us a fairly good 
idea of the shell that left the impressions. 
Inoceramya concentrica sp. nov. 
pi. xii, figs, i, 2 ; pi. xiii, fig. 1. 
Shell broad-ovate, slightly oblique, with the hinge margin long and 
straight; anterior cardinal margin probably nearly rectangular, post¬ 
cardinal margin sharply rounded and forming a wider angle ; anterior 
and ventral portions of outline nearly semicircular. Valves depressed 
convex; beaks small, situated anterior to the center; umbonal ridge in¬ 
conspicuous. Surface concentrically waved, the average width of the 
undulations increasing with age from 1 mm. or 2 mm. on the umbones 
to 4 mm. or 5 mm. on the central and ventral parts of adult shells. 
The concentric undulations do not cross the compressed elongate tri¬ 
angular posterior wing, but cease along a line separating the wing 
from the body of the valve. The wing itself is marked by much finer 
and rather obscure striae. Hinge plate wide just beneath the beaks, 
where a specimen broken off at this point (see pi. xn, fig. 2) shows 
two long vertical ligamentary pits and behind these a series of shorter 
and gradually diminishing pits that may be traced beyond the middle 
of the distance to the post-cardinal extremity. Immediately beneath 
this pitted margin there is a narrow depression, becoming obsolete pos¬ 
teriorly, and beneath this the heavy, posteriorly widening interior rib 
marking the separation of the wing from the body of the shell. This 
begins just behind the beak and dies out as it widens, becoming quite 
obsolete before reaching the posterior margin. Muscular scars and 
pallial line not observed. 
Dr. W. H. Dali, who discovered the specimens above described, 
refers to the species as i apparently a Posidonomya ’ in his Report on 
Coal and Lignite of Alaska (page 872 of the Seventeenth Annual Re¬ 
port of the United States Geological Survey). Though we have shown 
already, in our remarks on the new genus that we have believed it 
necessary to establish for their reception, why they should not be 
referred to Posidonomya , we may repeat that besides the ligamentary 
pits, which are absent in true species of that genus, the internal rib in 
