Professor R. Burckhardt — On Hijperodapedon Gordoni. 531 



been confined to this particular place is, that the mandibular teeth are 

 in correlation with the presence of a true horned beak, which would 

 have been a natural obstacle to any development of teeth. In this 

 way, and further too by the continuous renewal of teeth, another 

 useful means has been contrived for the process of mastication. 



By way of compensation, during the individual development the 

 dentulous surface was so modified that no disadvantage would accrue 

 to the animal through the incapacity of the mucous membrane to 

 remove the worn-down acrodont teeth. 



To look for a physiological parallel of this peculiar kind of 

 dentition it is necessary to go to the Placodontia. Of these latter, 

 however, the material at my disposal did not permit of a detailed 

 comparison, but it should be mentioned here that in the Placodonts 

 the maxillary appears to me to bear no teeth, and to be divided from 

 the tooth-row by a suture. 



Eeferring specially to the tendency to expansion, Avithin the 

 dentigerous portion of the posterior part of the lower jaw, by the 

 introduction of new elements, I remember a pai'allel case, having 

 noticed on the vomer-plate of a species of Pycnodon, which is in 

 the Museum at Basle, that the otherwise regular quinto - serial 

 arrangement was augmented by an additional tooth in its widest 

 posterior part. 



From the character of the teeth in Byperodapedon, the question 

 arises once more as to the natural haunts of the animal. What 

 is the kind of food required in the case of a placodont animal ? 

 Generally this consists of Crustacea, Molluscs, Echinoderms, and 

 other hard-shelled animals. If we take into consideration the 

 dentition alone, coupled with the extraordinary position of the eyes, 

 to which may be added the reduction in size of the posterior 

 extremities, one feels inclined to attribute to it a marine existence ; 

 more particularly would this be the case if Huxley's supposition 

 as regards the length of its tail were confirmed. On the other hand, 

 the structure of the manus, in which no tendency to a lengthening 

 of the phalanges can be perceived, is in direct contradiction to this 

 interpretation. 



ITyperodapedon, therefoi'e, and probably also Rhynchosanrtis, will 

 have to be regarded as inhabitants of the littoral. What other 

 terrestrial animal is equipped with a similar dental structure ? Or 

 what else could have induced Hyperodapedon to frequent the sandy 

 Triassic shores, from whose strata up to the present no signs of 

 petrefactions have been procured, except fossil reptiles.^ 



' The M-riter desires to refer to the remarkable discoveries made prior to 1892 

 iu the El^in Sandstone, Morayshire, which were described by !Mr. E. T. Newton, 

 F.R.S. (Proc. Eoy. Soc, Dec. 15, 1892, vol. lii, pp. 389-391 ; Phil. Trans., 

 vol. clxxxiv) , in which he enumerates Gordonia Traquairi, G. Hnxkyiana, G. Buffiana, 

 G. Judcliana, Geikia Elginensis, and Elginia mirahilis. Reference was also made 

 to a form resembling JEfosaur/is (Geol. Mag., 1893, p. 557) named Ori/ithosuchiis 

 Woodivardi, and to Erpetomchus Granti (see Proc. Roy. Soc, Dec. 7, 1893, 

 vol. liv, pp. 437, 438 ; Phil. Trans., 1894, vol. clxsxv B, p. 573, pis. liii-lvi). 

 There were probably also two s])ecies of Thecodontosauriis from the Trias of Bristol, 

 and perhaps a third from Leamington. 



