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. 1,2; pi. xni, fig. I. 



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 i 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. xii, 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. Dall, who discovered the specimens above described, 

 refers to the species as ' 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 



