. Bulletin of the British Museum (Natural History), Geology. 80 MORRIS, DICKINS AND ASTAFIEVA-URBAITIS. Fig. 29 Praeundulomya maxima (Portlock). Upper Carboniferous, Upper Pennsylvanian, Cisco Formation, near Jacksboro, Texas. BM PL5012. C. H. C. Brunton Collection (previously identified as subcuneatus Hall); Fig. 29a, dorsal view; Fig. 29b, side view; Fig. 29c, anterior view; all xl-5. Runnegar (1974) also took this view and rejected the name as a nomen dubium. However, our more recent examination of the type material of Allorisma barringtoni Thomas, 1928, first described as Carboniferous but
. Bulletin of the British Museum (Natural History), Geology. 80 MORRIS, DICKINS AND ASTAFIEVA-URBAITIS. Fig. 29 Praeundulomya maxima (Portlock). Upper Carboniferous, Upper Pennsylvanian, Cisco Formation, near Jacksboro, Texas. BM PL5012. C. H. C. Brunton Collection (previously identified as subcuneatus Hall); Fig. 29a, dorsal view; Fig. 29b, side view; Fig. 29c, anterior view; all xl-5. Runnegar (1974) also took this view and rejected the name as a nomen dubium. However, our more recent examination of the type material of Allorisma barringtoni Thomas, 1928, first described as Carboniferous but here reinterpreted as Upper Artinskian, has shown us that there really are some species that have shells much shorter than Undulomya maxima, and which seem to be intermediate in form between Undulomyinae and the Mesozoic species of Homomya and Pholadomya. They also show no trace of the internal dorsal posterior ribs that are typical of Undulomya and Praeundulomya. For these reasons we resurrect the generic name Exochorhynchus. Exochorhynchus barringtoni (Thomas, 1928) Figs 30a-b 1928 Allorisma barringtoni Thomas: 221-2; pi. 7, figs 5, 6. ?1960 Allorisma similis Lutkevich & Lobanova: 83; pi. 10, figs 6-8. Material. The holotype is SM A4948 (Fig. 30), and SM A4971 is one of several paratypes in the same collection; all are apparently from the Permian, probably the Upper Artinskian, of Sullana Rd., 1-5 miles south of El Muerto and Steel Hill, Parinas Quebrada, NW Peru, but (?wrongly) associated with mid-Pennsylvanian ammonoids when they were collected. Description. A medium-sized species with rounded posterior and anterior margins and the ventral margin sub-parallel to the hinge line. The regular, low comarginal rugae are very similar to those of Praeundulomya. The posterior gape is very narrow and the narrow escutcheon is defined by a low, but distinct, carina. The umbones are well to the anterior, and the shell is apparently very thin. Remarks. Exocorhynchus barringtoni res
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