188/.] TELEOSTEAN GENUS RHACOr.KPIS. 541 



fill dentition ; Hemielopopsis ' seems to be distingnished, 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. Thrissopater^ differs in having a 

 compressed abdomen, while Halec * and Halecopsis ' are too im- 

 perfectly 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 esta- 

 blished". 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 con- 

 stitutes a decided link between the old bony Ganoids and fishes of 

 a more modern type. 



EXPLANATION OF THE PLATES. 



Plate XL VI. 



Fig. 1. Rhacolepis brama, § nat. size. [P 3986.] 



2. Rhacolepis 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 6.] 



" F. Bassani, " Descrizione dei Pesci Fossili di Lesina," Denkschr. kais. Akad. 

 Wiss. vol. xlv. (1882), p. 215. 



2 G. C. Laube, " Beitr. Kennt. Fische bohm. Turon's," Denkschr. k. Akad. 

 Wiss. vol. 1. (1885), p. 286. 



^ A. Giinther, " Figs. & Descr. Brit. Organic Remains,'' Mem. Geol. Surv. 

 dec. xiii. pi. i. 



^ L. Agassiz, Eech. Poiss. Foss. vol. t. j)t. ii. p. 123. 



^ Undeseribed specimens in British Museum. 



^ The fossiliferous nodules also contain numerous individuals of a species of 

 entomostracan, but this, imfortunately, 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 pro- 

 bably 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 Clialk and 

 Tertiaries." 



Proc. Zool. Soc— 1887, No. XXXVI. 36 



