1893.] ON THE 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 



From these measurements it is evident that the principal teeth 

 in the lower jaw are about eight times, tbose 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 discoA'erer. In 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 trans- 

 verse 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 the size of Myliohutis joentoni ; but if the 

 few small examples of the recent Myliohatis 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 

 live metres. 



EXPLANATION OF PLATE XLVIII. 



Fig. 1. Myliobatis pentoni, sp. nov. ; portion of upper dentition, two-thirds nat. 

 size. LoTver Tertiary, Mokattam Hills, Cairo. 1 a. TransTerse sec- 

 tion of crown of median tooth. 

 2. Ditto; portion of lower dentition of same fish, two-thirds nat. size. 

 2a, Transverse section of crown of median tooth. 

 The original specimen was presented to the British Museum by Surgeon- 

 Captain R. H. Penton, June 18'J3. 



4. On the Cranial Osteology of the Mesozoic Ganoid Fishes, 

 Lepidotus and Dapedius. By A. Smith Woodward, 

 F.Z.S. 



[EeceiTed June 20, 1893.] 



(Plates XLIX. & L.) 



One of the earliest attempts to describe the skeleton of a Lower 

 Mesozoic fish w as based upon Lejndoius ', and the same genus 

 still affords the most satisfactory material for determining the 



^ F. A. Quenstedt. ' Veher Lejndot us im Lias Wiirttembergs.' 4to, Tiibingen, 

 1847. 



