GANOCEPHALA. 199 



aperture, and mesially throughout a great part of its extent 

 the outer boundary of the great palatal vacuity. It supports 

 a row of teeth, of which one or two at the fore part are of 

 large size. 



Between the orbit and the maxillary extends the bone (26) 

 which agrees with the malar of the crocodile, and with the 

 suborbital bones of fishes. 



The bone (27) answers to the squamosal in the crocodile, 

 but is chiefly a dermal ossification. It indicates, with the 

 supra-squamosal, the tendency to excessive dermal ossification 

 of the skull, and the " postorbital" corresponds in position with 

 the posterior suborbital scale-bones in Amia and Lepidosteus. 



The hinder angles of the skull are formed by the tympanic ; 

 in young individuals the tympanic does not extend backward 

 beyond the par-occipital, but as age advances it projects 

 further backward. It appears to abut internally against the 

 pterygoid. 



The two rami of the mandible were loosely united at a 

 short symphysis. The angular element (30) presents a con- 

 vexity answering to the point of ossification whence some faint 

 ridges radiate upon its outer surface. The dentary (32), if it 

 does not form the articular surface, begins very near it, and 

 each ramus appears to be composed of these two bones. 



From fishes the lower jaw of Archegosaurus differs in the 

 great length or forward extension of the angular piece (30) ; 

 but it resembles the piscine type in the simplicity of its 

 composition. The angular piece is, however, longer in the 

 Ganoids — e.g., Amia, Polypterus, Lepidosteus, — than in other 

 fishes ; in Lepidosiren its proportions are almost those of the 

 Archegosaurus; and it offers similar proportions in the man- 

 dible of the Axolotl and Proteus (fig. 84). 



The teeth in Archegosaurus have the simple conical pointed 

 shape. They are implanted in the premaxillary, maxillary, 

 mandibular, and vomerine bone, and in a single row in each. 



