CONTENTS. Page. The Occurrence of Cardita planicosta Lamarck in Western Oregon. 7 10 Unionida 13 Helicida Supplementary Notes on the Non-Marine Fossil Mollusca of North America.......... 14 17 ON FOSSIL MOLLUSCA OF WESTERN NORTH AMERICA. By CHARLES A. WHITE THE OCCURRENCE OF CARDITA PLANICOSTA LAMARCK IN WESTERN OREGON. There is probably no species among all the known fossil mollusca concerning the geological age of which paleontologists are more in accord than Cardita planicosta Lamarck. It is a well-known form in the European Eocene, and it is quite as characteristic of the Eocene of our Atlantic and Gulf borders as of the Eocene of Europe. The European and the American specimens differ so little from each other that no one now pretends to question their specific identity. Both the European and the American strata furnish specimens of this species in abun dance and great perfection, so that all the characteristics of the shell are well known. Its range of variation is not great, being mainly apparent in the extent of the obliquity of the axis and in the varying distinctness of definition of the ribs and their interspaces. In some cases the ribs upon the anterior part of the shell are broadly convex and the interspaces there are narrow and not sharply defined from the ribs at their sides. In the case of a few examples these characteristics are found to extend to other parts of the shell, but usually the ribs are more flattened and the interspaces are in the form of more or less sharply defined grooves between the ribs. In some cases the ribs and separating grooves are distinctly defined all the way from the beaks to the free borders of the valves; but in others the ribs and grooves, while they are well defined upon the umbonal and median portions of the valves, become obsolete before reaching their free borders. The character of the variation to which the surface markings of this species are thus seen to be subject is such that if it becomes excessive in any group of examples it is suggestive of a specific difference. The hinge is massive and constant in its characteristics. Mr. Conrad recognized this species among the collections which were obtained some thirty years ago, by parties connected with the Pacific Railroad surveys,1 from strata in California which have since received 1 See Pacific Railroad Reports, Vol. V, pp. 317-329. the name of the Téjon group. This group was referred to the Cretaceous period by the geologists of the State Geological Survey of California, but I have elsewhere given my reasons for regarding it as of Eocene age, as Conrad, Marcou,3 Heilprin, and others have also done. The California Cardita just referred to, Mr. Gabb regarded as specifi cally distinct from C. planicosta, and he gave it the name of C. hornii. I have seen only fragments of the California form, but judging from these and Mr. Gabb's description and figures I am inclined to regard it as a variety of Cardita planicosta, differing from the typical form only in the unusual broad convexity of all, instead of a part, of its ribs. The difference between the California specimens and the typical form of C. planicosta, as was shown by Mr. Gabb, consists mainly or wholly in this character of the ribs and their interspaces, and this I believe to be a case of extreme variation of Cardita planicosta which is not incompatible with specific identity. Some specimens of Cardita which I am unable to distinguish specifically from C. planicosta have been lately received at the office of the Survey which were obtained from stráta that are perhaps equivalent with those which bear C. hornii, and which appear to me to be intermediate between typical examples of that species and C. hornii. These specimens were collected by Prof. Thomas Condon, at the town of Albany, in the Willamette Valley, Oregon. They are numbered 13405 in the Catalogue of the U. S. National Museum. The stratum which contains these shells is an indurated dark colored shale, which was found a few feet beneath the surface of the ground in digging a cistern at Albany. Unfortunately neither this stratum nor any known equivalent of it has been recognized elsewhere, and as the stratum was not exposed at the surface where the discovery was made, and the digging was not continued through it, nothing is definitely known of its stratigraphical relations. Strata which bear characteristic Miocene fossils are found in the valley of the Willamette only a few miles away, but so far as I am aware no strata have hitherto been found in any part of Oregon which bear Cardita planicosta, nor any nearly related form. Unfortunately, also, although a dozen or more of the shells of the Cardita were ob tained, only a few fragments of other shells were found associated with them. Some of these apparently represent a species of Crassatella, but further than this they have yielded no definite information. Three of the valves found at Albany are represented on Plate I. These specimens have been chiseled out of the matrix in which they were embedded, and their natural surface has therefore been somewhat injured, but enough remains in its natural condition to give a fair illus 2 See Bull. U. S. Geol. Survey, No. 15. See Bull. de la Société Géologique de France (3), T. XI, 1883, pp. 407-435. 4 Proc. Acad. Nat. Sci. Phila. for 1882, pp. 195-214; also Cont. Ter. Geol. and Paleont. U. S., pp. 102–116. "Paleontology of California, I, p. 174, Pl. 24, Fig. 157; also II, p. 187, Pl. 30, Fig. 83. tration of the surface characters of the shell. The vertical diameter of two of these illustrated specimens is proportionally less as compared with the transverse diameter, than is usual with typical examples of C. planicosta, but that of one of them is not, and several of the other associated specimens have also the usual dimensions. None of the Oregon specimens show the hinge in a sufficiently perfect condition to afford a good figure of it, but some of them exhibit the hinge characters so plainly as to make it evident that this far western shell does not differ in respect to the hinge from the typical examples of Cardita planicosta. I do not therefore hesitate to refer it to that species. The discovery of these shells in Western Oregon possesses more than ordinary interest for several reasons. First, it adds materially to our knowledge of the already known wide geographical range of Cardita planicosta. Second, it is (if we except the Cardita hornii of Gabb) the first recognition of that species in the Pacific coast region that has been made. Third, Cardita planicosta being regarded by all paleontologists as of Eocene age, its discovery in Oregon indicates the existence there of the Eocene formation, of which no other indication is now known. It is not improbable that the stratum from which these fossils were taken represents a portion of the Téjon group of California. No evidence of it, however, is at present known other than that which is suggested in the preceding paragraphs and by the probability that the Oregon specimens and the C. hornii of Gabb are only varieties of Cardita planicosta. (173) |