536 MR. A. SMITH WOODWARD ON THE [June 23, 



still more recently Professor Cope ' 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 S|)ix and Martius. 



Tliough for the most part beautifully preserved, the specimens 

 present the usual imperfections so embarrassing in palBeontological 

 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 Mhacolepis had 

 a very slightly compressed form, without abdominal " keel," some- 

 times much elongated (as in R. biicca/is), 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 XLVI. 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. Tliere 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 premaxillae, 



' E. D. Cope, " On two extinct forms of Physostomi of the Neotropical 

 Region," Proc. Amer. riiil. Sue. vol. xii. (1871). p. 53. 



