Actinopterygii Protospondyli. 



105 



the lower margin of the trunk. In several genera {e.g. Meso- 

 don, Microdon, and Coelodus^ the tail is destitute of scales. 

 These fishes range from the Lower Lias (Mesodon liassicus) 

 to the Upper Eocene (Pycnodus platessus) with very little 

 modification. The fine series of examples of Gyrodus from 



FIG. 149. Portions of Pycnodonts. a. transverse section of jaws, showing the two 

 halves of the mandibular dentition opposing the vomerine teeth; b, dentition of 

 Microdon; c, dentition of Coelodus ; </, portion of vertebral column of Ccelo<!us, 

 showing persistent notochord (shaded), and the expanded bases of the arches; 

 < J , the same of Pycnodux ; /, inner view of scales, showing mode of interlocking by 

 pegs and sockets, which are continued as longitudinal ribs. 



the Lithographic Stone of Bavaria, and of Palceobalistuiu 

 from the Hard Chalk of Mount Lebanon, are particularly 

 worthy of attention. The armoured Coccodus and Xenopliolis 

 from Mount Lebanon are also remarkable. None of these 

 fishes have vertebrae, but in the later genera the arches above 

 and below the notochord are often expanded to unite at the side 

 (Fig. 149). 



The Eugnathidse are the rhombic-scaled forerunners of the Wall 

 modern A-mia (Fig. 150), and range from the Upper Trias or Nos 

 Rhaetic to the Chalk. They are predaceous fishes with a large rp "- K 



-cases, 

 12, 13,' 



FIG. 151. Eugna.thus orthosto-.nus, Ag. ; Lower Lias, Lyme Regis. 



