292 Mr. J. W. Davis on the 



has been lent to me by Professor John Young- ; and for the 

 second I am indebted to Mr. James Thomson, of the same 

 city. The former, represented on PI. XIII. fig. 1, exhibits 

 the anterior portion of the body of the fish ; the head, con- 

 sisting of a mass of cartilaginous or chondroid substance, 

 occupies nearly one half of the part preserved. The mouth, 

 with teeth scarcely at all displaced, is well defined, the man- 

 dible is large, the upper jaw is not so easily distinguishable 

 from the other elements of the cranium ; the anterior extre- 

 mity of the snout is unfortunately absent, but sufficient 

 remains to show that the head was large and broad. A 

 hollow above the posterior teeth of the jaws may indicate the 

 position of the orbit. The whole of the surface of the head, 

 together with the remainder of the body, is covered with 

 glistening dermal tubercles or shagreen. The teeth are 

 numerous, and, so far as can be identified, arc arranged in 

 shark-like concentric rows. Those occupying the posterior 

 surface of the jaws are the teeth hitherto known as Pleurodus, 

 whilst the anterior teeth, far larger in number, are those 

 styled Helodus or Lophodus. The front teeth are pointed and 

 adapted for seizing and holding prey, whilst those behind 

 gradually assume broader and more massive proportions, and 

 apparently in the palatal teeth of the Pleurodont type there is 

 evidence of the ankylosing of three or four teeth together. 

 The teeth occupying a median position in the jaws have their 

 longer axis in the same line as that of the jaw, with the 

 result that the Lophodont or Helodont teeth present an ex- 

 ternal cutting-surface, which resembles, when a pair of teeth 

 is taken separately, the dentition of some of the Petalodonts 

 (fig. la). The head, viewed from the front side, where the 

 matrix is fractured, is seen to be squeezed over towards the 

 exposed surface, and the opposite rami of the jaws can be 

 traced along the edge of the matrix. The length of the rami 

 of the jaws is 0*03 m. ; at a distance of 0*025 in. behind the 

 extremity of the jaw is a spine which has apparently been 

 displaced ; it is 0*040 in. in length and 0"010 m. in breadth, 

 and, pointing towards the head, the spine extends in a dia- 

 gonal direction with the base towards the dorsal aspect of the 

 fish. Its position appears to indicate .that it was located 

 immediately behind the occipital region of the head. 



Mr. Thomson's specimen does not exhibit the teeth in situ 

 in the jaws, but in a slightly segregated form on the slab ; 

 in this respect it forms an extremely valuable companion to 

 the specimen already referred to, because the relative size and 

 form of the teeth are better seen. There are six teeth exposed 

 of the Pleurodont type and near sixty teeth may be counted 



