626
Journal of Paleontology 92(4):611–633
Description.—Large-sized, 70–100mm long elongate shell, widely inflated; anterior margin broadly rounded, ventral margin broadly curved, posterior margin narrowly rounded, merging with dorsal margin that slopes up to umbo. Umbos broadly incurved, slightly prosogyrate; no lunule; escutcheon wide, shallow, merging smoothly with valve margins. Orna- mented by rounded growth rugae wider than interspaces, widening and diverging posteriorly.
Remarks.—Perkins (1961) distinguished H. vulgaris from his new species Homomya auroraensis by several relative differ- ences in shape: (1) the dorsal margin of H. auroraensis is more concave than H. vulgaris; (2) the posterior margin of H. auroraensis is more rounded; (3) beaks of H. auroraensis are more posterior from the anterior margin and less prosogyrate; and (4) the height-to-thickness ratio of H. auroraensis is less. These differences are subject to compaction and distortion. Only the latter two properties can be quantified and the differences are slight based on the small number of specimens. In a subsequent section, H. auroraensis is considered a junior synonym of Homomya budaensis. H. vulgaris occurs with Homomya budaensis in the Buda
Limestone in the Austin, Texas, area. They differ, however, by the valve outlines. H. vulgaris is an elongate shell and its dorsal margin is subparallel with the ventral margin. H. budaensis has a concave dorsal margin and its posterior is expanded and flares dorsally. This difference suggests that soft parts such as siphons or gills were larger in H. budaensis.
Homomya bravoensis Böse, 1910 Figure 10.4
1910 Homomya bravoensis Böse, p. 136, pl. 29, figs. 5, 6. 1928 Homomya bravoensis; Adkins, p. 141. 1961 Homomya bravoensis; Perkins, p. 83.
2002 Homomya bravoensis; Akers and Akers, p. 367, p. 368, fig. 350.
Holotype.—Homomya bravoensis holotype IGM 384, Colección Nacional de Paleontología (IGM), Universidad Nacional Autónoma de México; upper Albian, Böse’s unit 3 with Ceratostreon [Exogyra] texana, along the Southern Pacific and Bisbee Railroad, near Rio Bravo, Chihuahua, Mexico (source:
http://documents.mx/documents/fosiles-tipo- mexicanos.html; image available at: http://www.unipaleo.
unam.mx/public/consulta_publica.php).
Diagnosis.—Elongate, ovate shell, beaks at anterior margin, posterior margin truncate, inclined, rounded, gaping; ornament of low, rounded concentric rugae.
Occurrence.—Type specimens were collected at Cerro de Muleros (now Cerro de Cristo Rey) next to the Southern Pacific and Bisbee railroad near the Rio Bravo, New Mexico, west of El Paso, Texas, in the Del Norte Formation. It is also known in the Edwards Formation west of Round Rock, Williamson, and in Bell and Parker counties, Texas, in Walnut to Goodland for- mations, Fredericksburg Group (Adkins, 1928). It is reported in the Benevides Formation in the southern part of the Quitman
Remarks.—Böse noted that the overall form of H. bravoensis is similar to that of Homomya vulgaris Shattuck, but distinguished it by the position of the umbo, which is flush with the anterior margin. The escutcheon is elongate, ovate, but covered in lime mud (personal communication, J. Alvarado Ortega, 2017, Nacional de Paleontología del Instituto de Geología de la UNAM). H. bravoensis is from strata of the same age as L. (Sergipemya) alta but differs by its low umbo and oblong form; L. alta is characterized by a tall umbo very close to the anterior margin. The length and height of L. alta are subequal but the types of H. bravoensis are longer than high. H. bravoensis is similar to H. budaensis but tends to be shorter relative to its height, and the distance of the beak to the anterior margin of H. bravoensis is less than 10% and that of H. budaensis is nearly 30%.
Homomya budaensis Whitney, 1911 Figure 10.5, 10.7, 10.10
1911 Homomya budaensis Whitney, p. 15, pl. 3, figs. 1, 2, pl. 4, figs. 1, 2.
1928 Homomya budaensis; Adkins, p. 141. 1961 Homomya budaensis; Perkins, p. 81, pl. 24, figs. 4, 6, 7.
Mountains (Albritton and Smith, 1965). It also was collected from the University Mesa Marl near Fort Stockton, I-10, Pecos County, Texas (NPL 74414). In Mexico, it is in the lower part of the Upper Aurora Limestone. It ranges from middle Albian to lower upper Albian.
Description.—According to Böse:
Shell moderately oval, transversally extended, very con- vex, posteriorly extended; anterior margin short, obliquely curved ventrally, posterior margin longer, regularly curved; dorsal margin nearly straight, long, subparallel with ventral margin which is slightly arcuate; umbo strong, elevated, globose, curved towards anterior and below; positioned nearly at the extreme anterior of the valve. The valves are open at the posterior margin. The ornament consists of undulations and concentric grooves. Dimensions: length 55mm, height 40mm, width of one valve 18.2mm.
Our species rather resembles Homomya vulgaris Shattuck in its general form, but is distinguished by the position of the umbos, which in our species are positioned very near the anterior margin and by the less extended form. The species is rather rare and is encountered only in the lower part of the Cretaceous deposits at Cerro Muleros; Stanton and Vaughan cited the same horizon of Pleuromya knowltoni Hill; we have not found this species at Cerro Muleros. (Freely translated from Böse, 1910, p. 136.)
According to Adkins:
Form roughly elongate rectangular, rounded, oblique, quite inflated, beaks subterminal, proportionately less tall and more inflated than H aff. ligeriensis; reduced con- centric lines. (Adkins, 1928, p. 142)
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