. Bulletins of American paleontology. Miocene Lamellibranchs: Mongin 311. Text-fig. 5. Lyropecten estrellauns (Conrad). Wild Horse Canyon, Calif. Miocene. Coll. USNM 3586. Hinge shows three teeth in the left valve and two in the right, a triangular fold inside the pit of the chondrophore. nodules on the ribs, and the palHal margin is waved. This is not the case with C. )}}adisouia which, therefore, is not a Kodipecten^ contrary to the opinion of paleontologists. This species can no longer be likened to the subgenus Gigantopecten Rovereto, 1899 {Macrochlam^i '^ 1897) widely
. Bulletins of American paleontology. Miocene Lamellibranchs: Mongin 311. Text-fig. 5. Lyropecten estrellauns (Conrad). Wild Horse Canyon, Calif. Miocene. Coll. USNM 3586. Hinge shows three teeth in the left valve and two in the right, a triangular fold inside the pit of the chondrophore. nodules on the ribs, and the palHal margin is waved. This is not the case with C. )}}adisouia which, therefore, is not a Kodipecten^ contrary to the opinion of paleontologists. This species can no longer be likened to the subgenus Gigantopecten Rovereto, 1899 {Macrochlam^i '^ 1897) widely distributed in Europe and northern Africa during the Miocene. Indeed, the European shells of Gigantopecten have well-marked cardinal teeth, no byssal notch, equal and straight ears, thick, plain and smooth ribs, quite contrary to C. niadisonia. It must be pointed out, also, that the left valve of Gigantopecten has a flattened, "thumbmarked" apex and growth marks such as are never seen on the American species. Summing up, the large C. niadison'ia is never encountered in any European subgenus. Kautsky (1925, p. 14) finds C. niadisonia close to Pecten brinni/ieli Nyst (;;; Gilbert, 1945, p. 57, pi. 3, fig. 12) which, however, belongs to another genus. Only the sculpture has a certain, although artificial, simi- larity. There are subspecies of C. niadisonia in the lower and upper zones of the Choptank formation: a. C. niadisonia sayana (Dall) (1898, p. 725, pi. 26, fig. 6) in Alum Bluffs beds and Chipola formation. According to authors this would be the ancestral form of the species; it has more threads m the interspaces, round and flattened ribs, and no delicate concentric sculpture like C. radians (Nyst). The number of ribs studied on 44 left valves and 47 right valves of the specimens figured by Dall (Coll. U. S. Nat. Mus.) and coming from Oak Grove, Florida, gives a polygon of frequency of which the mode is 15 for the left valve and 16 for the right (Text-fig. P II).
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