100 MESSRS. HANCOCK AND ATTHEY ON 



others it is wanting. When the tooth is perfect its walls are 

 thick in proportion to the calibre of the pulp-cavity. The calci- 

 gerous tubes are very fine and numerous. 



Note. — After the above description of the tooth of Palceonis- 

 cus, it is scarcely necessary to say that there is no character by 

 which it can be distinguished from that of the so-called genus 

 Ganacrodus of Professor Owen (pi. 6) : the teeth of the latter 

 and former agree in size, form, and structure. We have found 

 the enamel tip to exist in P. comtus and other species from the 

 Marl-slate, as well as in the species from our Coal-Measures. 

 This we have proved in the most satisfactory manner, not by 

 taking the teeth at random as they are scattered through the 

 matrix, hut by taking the jaws from the heads of well authenti- 

 cated PalcBonisci, and examining the teeth both externally and 

 in section. After having done this in a great number of speci- 

 mens, we are enabled to state that the small enamel-tipped teeth 

 found detached in the Cramlington and Newsham shales are ex- 

 actly the same as those attached to the jaws. They are of the 

 same size and form, with the same bright tip of enamel and finely 

 fretted walls ; and in section there is no difference whatever, the 

 general form, the enamel-cap, the pulp-cavity, and dentine are 

 all precisely the same ; and all precisely agree with the tooth of 

 the so-called Ganacrodus. It is, therefore, hard to understand 

 what is meant by the use of such terms as "the villiform teeth of 

 Amblypterus and PaltBoniscus," "the vague and ill-defined cha- 

 racters of those en hrosse of Palceoniscus and ximhly icterus: " such 

 expressions may indeed mislead, as they or similar words ap- 

 pear to have misled their author, but they can never for a mo- 

 ment obscure the light derived from a thorough examination of 

 the facts. 



The laniary teeth of Palceoniscus and Amblypterus agree in all 

 essential characters ; and the tooth of the former is in every re- 

 spect similar to that of Professor Owen's "new genus." Conse- 

 quently this genus can never be adopted by palaeontologists. 



With regard to the coating of enamel on the crown of the 

 tooth, on which much stress is attempted to be laid, we can only 



