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Journal of Paleontology 92(4):611–633
Tamaulipas, Mexico. Imlay (1937) tentatively correlated this limestone with the Valanginian Stage by associated ammonites, which in Cuba are Valanginian (Myczynski, 1977). Pleuromya orbigniana (Rouillier, 1845) is in the
Berriasian-Valanginian Spilsby Sandstone in northeastern England near the North Sea coast (Woods, 1909, p. 256–257). This species is much smaller than H. knowltoni and is ornamented by either radial striae or radial rows of nodes. A similar species, Pleuromya peregrina (d’Orbigny, 1845), is found in lowermost Cretaceous near Moscow. Pleuromya hidensis Hayami (1959) in Aptian strata of Japan and eastern China is smaller than H. knowltoni and taller relative to its length, with a length-to-height ratio of 1.5. Pholadomya marcoui Cragin (1905) in the lowermost
Cretaceous Malone Formation in southwest Texas is very similar in form to H. knowltoni except for the six faint radial ribs (Akers and Akers, 2002, p. 359–360, fig. 342). If these species are not related they are convergent morphotypes. Pleuromya ingramensis Pampe (1975) is an internal cast
from the Middle Albian Basal Nodular Member of the Fort Terrett Formation, Fredericksburg Group, north of Kerrville, Texas. It is not a pleuromyid or pholadomyid. Its raised posterodorsal margin along the hinge line and pronounced grooves along the dorsal side are characteristic of arcid taxa such as Cucullaea.
Homomya comalensis Whitney, 1937 Figures 9.17–9.19, 11.11
1937 Homomya comalensis Whitney, p. 130, pl. 13, figs. 3, 4.
2002 Homomya comalensis; Akers and Akers, p. 367, p. 368, fig. 350, p. 468.
Lectotype.—The single specimen illustrated by Whitney would be the holotype, but has not been found in the collections of the Non-vertebrate Laboratory, the University of Texas. Consequently, a lectotype is selected from among two specimens recovered from the Regional Dense Member of the Person Formation in the road cut at Stone Crossing, New Braunfels, Texas (Porter, 2017), Texas Memorial Museum Non-vertebrate Paleontology Laboratory NPL85566. Unnum- bered paralectoypes are from the Regional Dense Member of the Person Formation at the exposures where highways Loop 1 and Loop 360 intersect in south Austin and from the basal Bee Cave Member of the Walnut Formation on the east road cut in Loop 360 at Bee Cave Road, Austin (Diehl, 2017).
Diagnosis.—A moderate-sized species 30–50mm long equi- valved, very inequilateral, ovate, globose valve, longer than high, slightly higher than wide; beaks low, slightly opisthogyrate, very close to rounded anterior margin, distance between beak and anterior margin averages 15% of the length; ventral margin broadly curved; posterior margin narrowly rounded, slightly gaping, and sloping into flat dorsal margin; no lunule, shallow wide escutcheon. Low, rounded concentric growth rings.
Occurrence.—Whitney’s specimen of Homomya comalensis is from the middle Glen Rose Formation: “About 275 feet (84 m) above the base … 2.3 miles (3.7 km) west of Fischer Store,
Comal County,” Central Texas (Whitney, 1937, p. 131). Newly discovered specimens are in lime mudstone from the middle Albian Regional Dense Member at the base of the Person Formation in Travis and Hays counties and in the Bee Cave Member of the Walnut Formation in Travis County. Total known range is lower to middle Albian.
Description.—Specimens are external casts showing exterior shape and ornament; most are incomplete. Beaks wide, very low, barely projecting above the straight dorsal margin. Ornament of weak, rounded, unequal, discontinuous concentric growth rugae, slightly steeper ventrally. Length ranges from36 to 55mm, height is from 19 to 35mm, and width is from 20 to 29mm. The distance from anterior margin to beak is from about 5mm to 11mm. Posterior gape is slight, not flared. Interior dentition, ligament, and musculature are unknown.
Remarks.—Whitney (1937) distinguished Homomya comalensis from H. knowltoni by its greater width, by its roun- ded gaping posterior margin, and by posterior ridges bordering escutcheon (Akers and Akers, 2002, fig. 230). H. comalensis is as long as smaller specimens of H. knowltoni, but its height is less and it is slightly wider at the same height than H. knowltoni and H. tarrantensis (Figs. 6, 8). The distance of the beak from the anterior margin of H. comalensis is less than of H. knowltoni margin so the beak-to-length and beak-to-height ratios are lower than for H. knowltoni (Fig. 8).
Homomya kellumi (Perkins, 1961) Figure 9.8–9.12
1910 Homomya aff.H. ligeriensis Böse, p. 137, pl. 29, figs. 1–4. 1928 Homomya aff. H. ligeriensis; Adkins, p. 142.
2002 Homomya aff. H. ligeriensis; Akers and Akers, p. 369, fig. 352.
1961 Homomya kellumi Perkins, p. 82, pl. 24, fig. 5; pl. 15, figs. 1–3, 5, 6.
1967 Homomya kellumi; Lokke, p. 352. 2002 Homomya kellumi; Akers and Akers, p. 468. 1961 Homomya tlahualiloensis Perkins, p. 83, pl. 25, figs. 4, 7–10.
Holotype.—Homomya kellumi: holotype UMMP 32855; paratypes: UMMP 32856, 32857. Type locality and age: Sierra Tlahualilo, localities 13, 14, 15, 18, 19, 31, and 51, Coahuila, Mexico, upper interval of Aurora Limestone, Lower Creta- ceous. Holotype of Homomya tlahualiloensis: UMMP 32858; paratypes UMMP 32859. Type locality and age: Sierra Tlahualilo, localities 14, 18, 26, and 29, Coahuila, Mexico, upper interval of Aurora Limestone, Lower Cretaceous.
Diagnosis.—A large-sized, 65–70mm long, equivalved species, very inequilateral, inflated, prosogyrate beaks 25% to 30% of the length from the anterior margin; anterior margin broadly rounded; ventral margin broadly arched toward narrowly curved posterior margin; dorsal margin nearly straight; gape extends anteriorly along dorsal commissure that merges into a low ridge up to beak.
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