Scott and Claggett—Albian pholadomyid bivalves Texas
Occurrence.—Upper Albian Main Street Limestone and lower Cenomanian Grayson Formation “in the abandoned D. B. and N. O. railway cut about half a mile southeast of the Union depot of Denison, Texas;…on summit of Pawpaw creek bluff southeast and east of Dennison” (Cragin, 1894, p. 60). Specimen WSA 10444 (Non-vertebrate Paleontology Labora- tory) was collected from a railroad cut in the Grayson Formation 0.5 mi southeast of the Union Depot, Denison, Grayson County, Texas.
Cragin’s Description.—“Shell large, curved-oblong, closed or nearly closed anteriorly and closed along the dorsal margin back of the beaks, obliquely truncated and gaping posteriorly, the breadth usually a little greater than the vertical dimension from hinge-margin to ventral margin, the length equal to somewhat more than one and a half times the breadth, the greatest breadth being about half way between the beaks and the mid-region; beaks nearly terminal, low, swollen, obtusely tangent, their bases long in the direction of the length of the shell and rising at a very low angle from their posterior origin to their rather broadly rounded summits; surface marked only with concentric growth-lines and undulations, and sometimes showing distally two or three broad growth-zones or stage” (Cragin, 1894, p. 59). Length 120mm, height 92mm, width 80mm.
Remarks.—A plaster cast of the incomplete holotype NMNH 32689 is curated in the Non-vertebrate Paleontology Laboratory at the University of Texas (WSA10444). The posterior portion is missing and the anterior, dorsal, and ventral margins are intact. A lunule is not developed because the umbo is terminal; the escutcheon is directly posterior to the beaks and is a narrow and shallow depression merging posteriorly with a straight commissure line. The concentric rugae are incompletely pre- served as flat ridges wider than the interspaces; the ribs slope slightly ventrally and widen from beak to ventral margin. Cragin (1894) originally spelled the species name
“washita,” which is an incorrect Latinization and therefore not an inadvertent error (ICZN, 2000 Code, Article 32.5, p. 39). The correct Latinized species name is “washitae” as a noun in the genitive case (ICZN, 2000, Code, Article 11.9, p. 13–14).
Homomya austinensis (Shattuck, 1903)
1903 Homomya austinensis Shattuck, p. 28, pl. 16, figs. 1–3. 1928 Homomya austinensis; Adkins, p. 141. 1961 Homomya austinensis; Perkins, p. 42. 2002 Homomya austinensis; Akers and Akers, p. 468.
Holotype.—Homomya austinensis holotype: NMNH PAL 315995; topotypes: NPL 62211, UT 30035, UT32168, UT 32299, UT 32308, UT 32309.
Diagnosis.—A moderately large, 50–65mm long, equivalved, inequilateral, ovate species about one and a half times long as
625
tall, relatively narrow width, weakly inflated, prosogyrate beaks 15% to 20% of the length from the anterior margin; anterior margin broadly rounded; ventral margin merging smoothly into curved posterior margin; dorsal margin nearly straight with a distinct escutcheon.
Occurrence.—Lower Cenomanian Buda Limestone, Austin, Texas.
Description.—Inflated, equivalved, inequilateral shell; anterior margin rounded, ventral margin broadly rounded, posterior margin narrowly rounded, dorsal margin slightly inclined toward beak, beak about one-fourth the length toward the anterior. Beaks nearly touching, slightly prosogyrate; no lunule, anterior hinge margin short; posterior hinge margin a low ridge at commissure, bordered by shallow groves that are separated from umbo by faint ridges. Slightly gaping. Ornamented by low, rounded, concentric growth rugae, wider than interspaces.
Remarks.—The outline and compact thickness of this species are more similar to H. kellumi than to other species in the Buda Limestone. Its escutcheon is shallow, fading out into the pos- terior gape and with a commissural pseudonymph ridge. Shat- tuck (1903, p. 28) differentiated H. austinensis from H. vulgaris in “that it is much shorter and broader in proportion to its thickness. It is also much more compressed.” H. austinensis is nearly indistinguishable from the French and Brazilian P. ligeriensis (d’Orbigny, 1845, in Ayoub-Hannaa et al., 2015). The length-to-height ratio of H. austinensis is 1.44 and that of P. ligeriensis is 1.52; H. austinensis is the larger species; the beak positions are virtually the same. P. ligeriensis is a cosmopolitan member of the Mediterranean to eastern South American provinces (Ayoub-Hannaa et al., 2015), and H. austinensis is a member of the Caribbean Province.
Homomya vulgaris Shattuck, 1903
1903 Homomya vulgaris Shattuck, p. 29, pl. 16, figs. 4–5. 1928 Homomya vulgaris; Adkins, p. 141.
2002 Homomya vulgaris; Akers and Akers, p. 371, 372, 469, fig. 354.
Holotype.—H. vulgaris holotype: NMNH PAL 315996; topo- types: NPL 62219, WSA 5013, UT 35380B.
Diagnosis.—An equivalved, very inflated, elongate shell much longer than tall, with subparallel dorsal and ventral margins; with a shallow, elongate escutcheon.
Occurrence.—Uppermost interval of the lower Cenomanian Buda Limestone, upper Washita Group in the Austin, Texas, area; in northern Mexico it is in the upper part of the upper Albian Aurora Limestone.
Figure 10. (1–3) Homomya washitae Cragin, 1894; plaster cast of WSA 10,444, incomplete specimen, Grayson Marl, Denison, Texas; (1) left valve; (2) dorsal view; (3) anterior view. (4) Homomya bravoensis Böse, holotype IGM 384, RV; (2017 image available at:
http://www.unipaleo.unam.mx/public/ consulta_publica.php). (5, 10) Homomya budaensis Whitney, 1911, hypotype UMMP 32854, collected by Perkins (1961); (5) left valve; (10) dorsal view. (6, 8, 9) Homomya cymbiformis Perkins, 1961; holotype UMMP 33012; (6) left valve; (8) right valve; (9) dorsal view. (7) Homomya auroraensis Perkins, 1961, holotype UMMP 32853, left valve. Scale bars=1 cm.
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