PLATE II. 



Ceratodus polymorphus — Mandibular Teeth. 



Figs. 

 1 — 13. Specimens from the Rhsetic of Aust Cliff, in the Bristol Museum. 



The figures illustrate the uncertain association of many characters which 

 have been relied upon as specific, such as breadth, angulation, prominence 

 of denticles or ridges, number of denticles, definition of external margin, 

 &c. The following series show noteworthy gradations : 



(1.) 5, 12, 3, 4, 1. 



(2.) 13, 11, 10, 7, 3, 2, 1. 



(3.) 13, 11, 6, 7, 3, 2, 1. 



Fig. 5 has no third denticle, but this form is connected with the rest by a tooth in the 

 Museum of Practical Geology (Xf), and this in turn by Fig. 12 of the present 

 Plate. Fig. 7 shows the attached splenial bone. Figs. 1, 2 have the first 

 denticle unusually prismatic. Figs. 11 and 13 are very broad ; 9 and 13 

 very flat. Fig. 10 has the external margin particularly defined. 



