1893.] 0>" TKE OSTEOLOGY OF LEPIDOTUS ASD DAPEBIUS, 559 



The following is a table of measuremeuts : — 



Upper Dentition. Lowek Dkntitiun. 



m. 111. in. m. 



Width of mediau deutal plates. . U-13 to U-135 U-12 to U-13 



Length „ „ • • 0-18 0-OU to 0-OlG 



Maximum ^^■idth of three lateral 



series of plates 0-02 0-02 



From these measurements it is evident that the principal teeth 

 in the loAver jaw are about eight times, those of the upper jaw 

 about seven and a half times as broad as long ; «"hile the maximum 

 thickness of the same teeth in both jaws equals nearly one-quarter 

 of their breadth. These characters, taken in conjunction \\ith the 

 form and proportions of the lateral teeth, suffice to distinguish the 

 Mokattam specimen from the dentition of all known species of 

 Mifliobatis ; and it may therefore be named M. pcntoni in honour 

 of its discoverer. In determining such specimens it is, of course, 

 necessary to take into account the mode of gro\Ath of the teeth and 

 their change in proportions with age ; but it does not appear 

 possible, in accordance with the ordinary laws of gro\Ath, 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- 

 vei'se section of the median teeth seem to be nearly constant at all 

 stages of growth in any one species. 



Materials for com])arison are unfortunately insufficient to form 

 any certain estimate of the size of Ilj/Iiobatis pentoni ; but if the 

 few small examples of the recent JMiiUohatis aquila in the British 

 Museum are at all similar in proportions, the maximum width of 

 the disk of the extinct species cannot have been mucli less than 

 five metres. 



EXPLANATION OF PLATE XLYIII. 



Fig. L Myliohatis pcntoni, sp. uov. ; portion of upper dentition, two-thirds nat. 

 size. Lower Tertiary, Molsattain Hills, Cairo. 1 a. Transverse sec- 

 tion of crown of median tooth. 

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

 2a. Transverse section of crown of median tooth. 

 The original specimen was presented to the Eritish Museum by Surgeou- 

 Captain E. H. Penton, June 1803. 



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

 Lepidottis and Dajjedius. By A. Smith Woodward, 

 F.Z.S. 



[Received June 20, 1893.] 



(Plates XLIX. & L.) 



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

 Me.so/.oic fish was based upon Lujfldotus\ and the same genus 

 still affords the most satisfactory material for determining the 



' I". A. Qucnstcdt. ' \I i:hcr Lqndofni^ ini Lias Wiu-l tcinbergs.' Ito, Tubinsjcn, 

 1^17. 



