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Show 536 MR. A. SMITH WOODWARD ON THE [June 23, still more recently Professor Cope1 has described an interesting physostomous fish from an uncertain locality in South America, which is not improbably derived from the same geological formation. This appears to be the complete literature of the subject, with the exception of brief allusions to the fossils to be noted later on, and none of the accounts are accompanied by figures, if we except the single imperfect drawing given by Spix and Martius. Though for the most part beautifully preserved, the specimens present the usual imperfections so embarrassing in palaeontological research. And while some show very little traces of crushing, or are merely laterally compressed, others were subjected to considerable disturbance before the surrounding mud and sand assumed a hardened state, and several are curiously distorted. There are some in a condition that may be appropriately described as " telescoped " -the fish having been apparently subjected to pressure at right angles to its long axis; and one specimen (B.M. no. 28616) is completely folded up in a most remarkable manner. Description. As proved by uncrushed individuals, the body of Rhacolepis had a very slightly compressed form, without abdominal " keel," sometimes much elongated (as in R. buccalis), and sometimes relatively short (R. latus). The roof of the skull exhibits a corresponding flatness, and the snout is remarkably acute. The paired fins are well developed, the pelvic pair being abdominal in situation; there is a single dorsal fin in the middle of the back opposite the pelves ; the anal is small, and halfway between the pelvics and the tail; and the caudal fin is deeply forked. The scales are small or of moderate size, and the lateral line is not apparent in unabraded specimens. Considering these various points in order, there are several features of interest in the head that first claim attention. In the cranium itself, a few of the elements can be more or less distinctly recognized, but the frontals alone are sufficiently perfect and characteristic to merit special note (Plate X L V I . fig. 3). They attain the usual large dimensions and are apparently united together throughout their entire length, the anterior half of the median suture being raised into a prominent longitudinal ridge, and the bone on either side of this for some distance exhibiting a very even surface. Posteriorly, in one specimen (B. M . no. P 1957), a pair of small rectangular bones are to be observed, meeting in the middle line, and these evidently represent the parietals. There is also another prominent element in some examples-as in the original of Plate XLVII. fig. 4-which may probably be interpreted as a membrane bone above the operculum, similar to that observed in certain Characinoids. The palatine bones are provided with teeth, as disclosed by a fracture in the skull of R. latus (no. P 1957); and both premaxilla?, 1 E. 1). Cope, " On two extinct forms of Physostomi of the Neotropical Region," Proc. Amer. Phil. Soc. vol. xii. (1871), p. 53. |