is relatively broad, ftisiform in shape, salad extends for one laalf to two thirds of the distance frona the valve apex, to the posterii- Or araargiai. tj'traaaraentaticaa COnsists Of StrOaag, reguiar, rnunded eoaaceaatric I tdges, Bboaat .5 per ataaii. separaled by intel'spaces 8 little Bart'rawel lhtna the Width ot 8 lldge. These Speoaaaaea1S 818y

red. satndstone litlaologies. in 1vhich it typically is the only fos- sill present. All ot the specaaaaens Bio pedacle vBlves. They Bre carcaJIlar lo shghtlv saiboval ln outline, anaan 8 naaxlinurn daaan- eter Ot 20 IBM. Bnd display 8 Slightly elevated BpeX tlaat lS cen- trally to posleriorly subcenlrally sitiniateal. Tbe pcdicle foramen

]S3

PERMIAN — TRIASSIC BOUNDARY AT EL ANTIMONIO, SONORA, MEXICO

be Orbicadoidea sp. 2 of Cooper (in Cooper et al., 1953), which is similar in valve size and outline. Direct comparison cannot be made, however, becaUse Cooper's specimens were brachial valves, whereas all of the specimens at hand are pedi- cle valves. These El Antinmnio specimens clearly differ from O. ovalis, described by Cloud (1944) from the Guadalupian of the I.as Delicias area, Coahuila, Mexico, which is a much slnaller species with a pronounced oval outline. Cooper and Grant (1974) did not report Orbicadoidea from the Permian of west Texas. The genus is known only from the Paleozoic (Rowell, 1965).

Fragmentary specimens identified as Spa'riferellina onoresnsis Cooper (Figure 38 D), o— ccur together with Composita sp. in thin lenses of shell debris within red, coarsely crystaHine limestone. Skeletal remains are typically coarsely recrystallized and locally severely weathered, precluding posi- tive identification of the specimens. The Spiriferellina shells are nearly twice as wide as long, attaining a maximum width of about 30 Inm. Each side of the valve bears up to nine, rounded prominent plications; the sulcus is relatively broad and includes one or more obscure radial costae. One specimen also displays the fine punctae characteristic of this genus (Figure 3D). The specimens of Composita are incomplete and cannot be identified to species. In addition to these brachiopods, an unidentifiable chonetoid brachiopod, a single unidentifiable gastropod steinkern, and moderate numbers of small crinoid skeletal elements were also collected from unit 8.