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Show 1893.] ON TUE OSTEOLOGY OF LEPIDOTUS AND DAPEDIUS. 559 The following is a table of measurements :- UPPER DENTITION. LOWER DENTITION. m. m. . m. m. Width of median dental plates.. 0-13 to 0-135 0-12 to 0-13 Length „ „ .. 0-18 0-014 to 0-016 Maximum width of three lateral series of plates 0-02 0-02 Erom these measurements it is evident that the principal teeth in the lower jaw are about eight times, those of the upper jaw about seven and a half times as broad as long ; while the maximum thickness of the same teeth in both jaws equals nearly one-quarter of their breadth. These characters, taken in conjunction with the form and proportions of the lateral teeth, suffice to distinguish the Mokattam specimen from the dentition of all known species of Myliobatis; and it may therefore be named M. pentoni in honour of its discoverer. Iu determining such specimens it is, of course, necessary to take into account the mode of growth of the teeth and their change in proportions with age ; but it does not appear possible, in accordance with the ordinary laws of growth, for any known type of dentition to develop by increase of size into the one now described. The form of the lateral teeth and the transverse section of the median teeth seem to be nearly constant at all stages of growth in any one species. Materials for comparison are unfortunately insufficient to form any certain estimate of tbe size of Myliobatis pentoni ; but if the few small examples of the recent Myliobatis aquila in the British Museum are at all similar in proportions, the maximum width of the disk of the extinct species cannot have been much less than five metres. EXPLANATION OF PLATE XLVIII. Fig. 1. Myliobatis pentoni, sp. nov.; portion of upper dentition, two-thirds nat. size. Lower Tertiary, Mokattam Hills, Cairo. 1 a. Transverse section of crown of median tooth.. 2. Ditto; portion of lower dentition of same fish, two-thirds nat. size. 2 a. Transverse section of crown of median tooth. Tbe original specimen was presented to the British Museum by Surgeon- Captain E. H. Penton, June 18(J3. 4. On the Cranial Osteology of the Mesozoic Ganoid Fishes, Lepidotus and Dapedius. By A. S M I T H W O O D W A R D, F.Z.S. [Received June 20,'1893.] (Plates XLIX. & L.) One of the earliest attempts to describe the skeleton of Mesozoic fish was based upon Lepidotus1, and the same genus still affords the most satisfactory material for determining the 1 F. A. Quenstedt, • Ueber Lepidotus im Lias Wiirttembergs.' 4to, Tubingen, 1847. |