Diastema, in some Fossil IteptiJes. 379 



enamel, badly preserved, appears to be wrinkled. Below the 

 middle of the canine tooth a shallow groove descends the 

 external lateral surface of the dentary bone. 



Behind the socket for the canine tooth a concave diastema 

 measuring- 1 ^^^ inch intervenes between that tooth and the 

 first tooth of the molar series. The crown of that molar 

 stands fully \ inch above the alveolar margin. It is sub- 

 quadrate, somewhat broken, less than \ incli in diameter, 

 with external and internal ridge-margins in front. It is worn 

 down transversely by apposition with a maxillary tooth to 

 make a flat grinding surface. Tlie posterior fracture ter- 

 minating the fragment of the ramus, [)asses vertically throu<'-h 

 the vacant socket for the second molar tooth, which is about 

 Y^ inch deep in the jaw and tapers as it descends. 



The diastema is the most interesting region of the Jaw, on 

 account of its length, for in Gomphognathus kannemeyeri the 

 concave interspace in the jaw between the mandibular canine 

 and molars measures less than j\ inch, which is less than 

 one-fourth as long as in this specimen. In Gomphognathus 

 polyphagus, in which the jaws are closed, the mandibular 

 diastema measures 1^^ inch. 



The region of the diastema is compressed from the outer to 

 the inner side, so as to make a blunt alveolar ridge situate 

 towards the flattened inner side of the ramus, wider behind 

 than in front. This ridge helps to detine the convexity of 

 the external suiface of tlie dentary bone. 



On carefully cleaning the summit-line it became evident 

 that the ridge of the diastema carries teeth. Their crowns 

 are level with the alveolar ridge or imperceptibly raised, and 

 give no indication of liaving been more elevated. Tliey have 

 the aspect of flattened ovate denticles each with a cential 

 depression, occupied with black matrix, situated in advance 

 of the molar teeth. They therefore appear to correspond in 

 position with the milk-teeth of mammals, in which the teeth 

 are shed and not replaced, but differ in being persistent in 

 the jaw and in their simple condition and small size. 



The teeth which are most evident are three in number, 

 raised above the bone by the thickness of a stout paper, and 

 defined at the base by a black line of matrix. It was neces- 

 sary to determine whether they were superficial ossifications. 

 I reluctantly sacrificed a part of the hindermost denticle, but 

 under the steel point the whole crown became dissipated, 

 displaying black matrix in the centre and an osseous rim. 

 On scraping away the matrix no doubt is left that thereof of 

 the tootli is still in its socket, n)argined externally by dense 

 white tissue, continuous with a minute fragment of the 



