2 



BRITISH FOSSILS. 



internal surface of the jaw, and extends from the crown to the base 

 (fig. 2, e.) In structure it closely resembles the analogous tritores in 

 the genus Edaphodon. Both the outer and inner surfaces of the 

 jaw, with the exception of this dentinal plate, were invested with a 

 layer of harder material, as in the genus Ischyodus. The remarkable 

 laminated structure, which suggested the generic name, obtains in 

 the upper as well as in the lower jaw. As in the other GhimcBToids, 

 Masmodus was furnished with two upper maxillary bones, and two 

 premaxillaries. The former were provided with three triturating 

 tubercles (fig. 6, a, b, c.) as in Edaphodon ; but, unlike those of the 

 latter genus, the dentine of which they are composed is confluent, 

 being rolled round, like a scroll, in the substance of the bone ; one 

 edge forming the margin of the tooth, the other buried deep in its 

 centre. In the young Masmodus (figs. 7, 8.) the two principal tubercles 

 were united at their bases. The premaxillaries (figs. 9, 10.) are thin, 

 incurved, scalpriform denticles, rounded at the cutting edge. They 

 have the lamelliform structure characteristic of this genus, but cor- 

 respond with the premaxillaries of the allied genera in the columnar 

 arrangement of the plates, although the columns are not separated 

 by septa of bone. 



History. — It was not likely that any organic body, exhibiting 

 peculiarities of structure such as are here detailed, should be passed 

 by unheeded by John Hunter. The specimens figured of the lower 

 maxillaries and the premaxillaries (figs. 1, 2, 9, 10.) are in the 

 Hunterian collection. Shortly after Dr. Buckland's ingenious dis- 

 covery of the afiinities of the fossil Chimceroids of the oolitic and 

 cretaceous formations, these specimens attracted the notice of Pro- 

 fessor Owen, who recognised their true character, and described 

 them under the name of Chimoera Runteri* In the course of my 

 examination of the Ghimceroid family in 1843, I proposed to elimi- 

 nate these forms under a separate generic title, an arrangement 

 which the subsequent discovery of the upper maxillaries fally con- 

 firms. The latter were found by the late Mr. Dixon at Brackles- 

 ham, and fortunately show the peculiarities of the dental arrange- 

 ment both in the young and the adult individual. 



Affinities. — The genus Masmodus approaches nearer to the recent 

 Ghimcera monstrosa than either Ischyodus or Edaphodon, in the 

 form and structure of the lower jaw, and the premaxillaries ; but 

 in every other respect its nearest affinity is with Edapliodon. 



Localities, — This species is found in the Eocene formations of the 



* Odontography, p. 66, 



