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Show 1887.] TELEOSTEAN GENUS RHACOLEPIS. 541 ful dentition ; Ilemielopopsis ' seems to be distinguished, among other features, by the absence of teeth on the margin of the mouth ; and Protelops' has relatively shorter and stouter jaws, with rounded crushing-teeth on the palatines. Thrissopater3 differs in having a compressed abdomen, while Halec4 and Halecopsis5 are too imperfectly known for certain reference. Geological Age. The foregoing conclusions become of especial interest when the geological antiquity of Rhacolepis is taken into consideration, for it almost certainly dates back to the Cretaceous period. As already shown by Agassiz, it is associated with other fossil fishes, e. g. the ganoids Aspidorhynchus and Lepidotus, and the teleostean Cladocyclus, the former of which are Jurassic and Cretaceous, and the latter Cretaceous, in European areas; and Gardner has recorded some remains probably of the cephalopod Turrilites from the same beds ; so that true homotaxis (geological contemporaneity) with the Upper Cretaceous formations of the Old World appears to be well established6. Rhacolepis is thus one more of the forerunners of the Teleostei, which seem to have become developed during Jurassic times, and to have swarmed in the Chalk seas: it is one which constitutes a decided link between the old bony Ganoids and fishes of a more modern type. EXPLANATION OF THE PLATES. PLATE XLVI. Fig. 1. Ehacolepis brama, § nat. size. [P 3986.] 2. Ehacolepis buccalis, anterior portion of fish. 2 a. Transverse section of trunk. [P 3983 a.] 3. Ditto, upper aspect of head. [P 1958.] 4. Ditto, vertebral centra, a. Side view. b. Section. [15793, P 1962.] 5. Ditto, posterior circumorbital bones. [P 1958 a.] 6. Ditto, scales, twice nat. size. [15485.] 7. Ditto, axillary appendage. [28900 b.] 1 F. Bassani, " Descrizione dei Pesci Fossili di Lesina," Denkschr. kais. Akad. Wiss. vol. xiv. (1882), p. 215. 2 G. C. Laube, "Beitr. Kennt. Fische bohm. Turon's," Denkschr. k. Akad. Wiss. vol. 1. (1885), p. 286. 3 A. Gunther, " Figs. & Descr. Brit. Organic Remains," Mem. Geol. Surv. dec. xiii. pl. i. 4 L. Agassiz, Rech. Poiss. Foss. vol. v. pt. ii. p. 123. 5 Undescribed specimens in British Museum. 6 The fossiliferous nodules also contain numerous individuals of a species of entomostracan, but this, unfortunately, does not assist in determining the precise age of the beds. It has been kindly examined by Professor Rupert Jones, F.R.S., and Mr. C. D. Sherborn, F.G.S., who regard the species as probably referable to Cytheridea. The former writes : " It differs from any species known to me, but in shape is near to C. perforata, Roemer, from the Chalk and Tertiaries." PROC. ZOOL. Soc-1887, No. XXXVI. 36 |
