The Permian Fishes of the Genus Acentrophorus*

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Abstract
Summary.: The propriety of placing Acentrophorus in the family Semionotidæ was not in doubt, but it is amply confirmed by the additional information now brought forward. In the exact correspondence of the dorsal and anal fin‐rays with their endoskeletal supports, in the reduced maxilla, the narrow preoperculum and the whole plan of the opercular apparatus, in the absence of an infra‐clavicle as in all the details of the pectoral arch and the pelvic bones, Acentrophorus is a characteristic representative of the Protospondyli and of the family Semionotidæ.At the same time, Acentrophorus does in, certain respects retain primitive characters. It seems, in fact, to provide us with something that is comparatively rarely found among fossil fishes, namely a really early representative of a new group which still shows definite traces of its origin from older types. The most striking character of this kind in Acentrophorus is undoubtedly the upper caudal lobe. As compared with the caudal lobe of the contemporary Palæoniscids it is certainly greatly reduced, but the reduction is only in depth; the caudal lobe of Acentrophorus is still of the maximum length. It is interesting to find in another contemporary fish, the extraordinary form Dorypterus hoffmanni, a caudal lobe in precisely the same stage of reduction, except that, in place of the single line of scales found in Acentrophorus, the lobe is marked externally to the very tip by two lines, the scales as they are traced backwards becoming linear and almost microscopic. That a stage such as this—the caudal lobe extremely attenuated in its hinder portion but still more or less of the full length—may have been passed through pretty regularly in the production of a “hemi‐heterocercal” tail is suggested not only by the parallelism between Acentrophorus and Dorypterus, but also by at least one fact of fossil ontogeny. A young example of Dapedius, 9 cm. long and beautifully preserved (text‐fig. 16), which has been kindly lent me by Prof, D. M. S. Watson, has a caudal lobe extending just two‐thirds the length of the upper border of the tail, instead of only one‐third as in the adult; and for some distance towards its apex the lobe is marked by a single line of very small and narrow scales.The head of Acentrophorus also, though in side view almost entirely typical of a Semionotid, shows when seen from above (text‐fig. 3) some decided primitive traits. The shape of the frontal shield and of the supratemporals, and the large leaf‐shaped post‐temporals, are all strongly reminiscent of Palæoniscus. So also is the internal pattern and articulation of the scales, especially in the earliest species, A. glaphyrus.On the other hand, there are certain points in the structure of Acentrophorus which seem to be peculiar to this genus among the Semionotidæ. The orbit is very large and encircled by only a single chain of bones; the scales, compared with those of other Semionotids, are thin, deeply overlapping and of unusual range of difference in size, and do not produce the typical “tessellated pavement” effect; the fin‐fulcra in the later species are of particularly massive form; the pectoral fin is placed higher on the flank than in other fusiform Semionotids, and its attachment gives it a peculiarly “modern” appearance. Fused frontals are also unusual in the family; and of the characters of the axial skeleton which are shown clearly enough to be worth discussion, the expanded neural spines under the dorsal fin and the great size of the supports of the anal fin‐rays seem to be quite peculiar to Acentrophorus.In view of all these primitive and individual characters it can no longer be said that Acentrophorus differs from Semionotus only in lacking the median row of acuminate scales in advance of the dorsal fin. It is seen to be very much the most primitive known member of the family Semionotidæ, and is therefore difficult to incorporate in any scheme on which the genera may be classified. On the whole, however, it would appear to be nearest to the line of ancestry of Lepidotus. Such a relationship is suggested in many points, in particular in the shape of most of the tins and in other details of their structure, in the excavation of the back along the base of the dorsal fin, and in the tendency towards a tritoral form exhibited by the teeth on the vomers.My thanks are due to Prof. D. M. S. Watson, who first suggested the desirability of re‐examining the material of Acentrophorus in the Hancock Museum at Newcastle; to Mr. J. A. Charlton Deas and Mr. T. R. Goddard for giving me facilities for studying some fine specimens in the Sunderland Museum; and to Dr. W. E. Collinge for similar privileges at York.

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