PYCNODONTS. 73 



grinding surface. They give off very regular, but extremely minute 

 branches, which are lost in the clear and dense enamel-like superficial 

 layer of the tooth. 



In the genus Microdon, the teeth, though presenting all the 

 general characters of the Pycnodontal structure, are reduced to their 

 smallest dimensions. They present a flattened angular form, and are 

 arranged in many rows on the intermaxillary, premandibular and 

 vomerine bones. The whole substance of the tooth is composed of 

 calcigerous tubes, which are straighter, more parallel, and relatively 

 finer than in Gyrodus, whence there results a texture of still greater 

 density, and one that approaches very closely to that of the molar teeth 

 of the Gilt-head (Chrysophrys). The arrangement of the component 

 tubes of the tooth of Microdon is shown in PI. 43, fig. 1 . 



The pavement of thick, round, convex or flattened teeth of the 

 genera of Pycnodonts, above described, was adapted, like the corres- 

 ponding teeth of existing fishes, to break and crush small testaceous 

 and crustaceous animals. These teeth, under the name of Bufonites, 

 occur most abundantly in the oolite formation. 



The teeth of the extinct fishes of the genus Placodus present 

 reverse proportions to those of the Microdon, and here attain their 

 maximum of development in the Pycnodont family. Plate 30, fig. 2 

 exhibits the alveoli of the prehensile intermaxillary teeth, and the 

 molar teeth of the same bone in situ of the Placodus Andriani, Ag. 

 The prehensile anterior teeth were arranged in two transverse rows 

 of six in each row. The anterior were the largest, and presented a 

 singularly elongated cylindrical form, with a conical slightly recurved 

 obtuse crown (fig. 3). In Placodus gigas, the crown of the incisor is 

 more expanded and more abruptly bent. These teeth, like the diver- 

 gent anterior teeth of the Wolf-fish, must have served to grapple with 

 large Testacea or Crustacea, the crushing and comminution of which 

 were then completed by the posterior dental plates. These are 

 arranged in four rows, of which the two external ones include each 

 four smaller subcircular teeth ; the two internal rows consist each of 

 three large tetragonal dental plates with the angles rounded off, and 

 the upper surface flattened and smooth. 



In the lower jaw it would seem that three similar dental plates in 



