GANOCEPHALA 177 



— e.g., Amia, Polypterus, Lcpidosteus, — than in other fishes ; 

 in Lepidosiren its proportions are almost those of the ArcJie- 

 gosaurus ; and it offers similar proportions in the mandible 

 of the Axolotl and Proteus (fig. 65). 



The teeth in Archegosawms have the simple conical pointed 

 shape. They are implanted in the prernaxillary, maxillary, 

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

 In the short premaxillaries there are from 8 (A. Decheni) to 

 12 (A. latirostris) ; they are rather larger than the maxillary 

 teeth. These follow in an unbroken series to beneath and 

 beyond the orbit, and are about 30 in number ; but their in- 

 terspaces are such as would lodge double that number in the 

 same extent of alveolar border. The vomerine teeth are in a 

 single row, parallel with and near to the maxillary row ; one 

 or two behind the choane are much larger than the rest, which 

 resemble the maxillary teeth in size. The mandibular teeth 

 extend backward to the coronoid rising, and decrease in size, 

 the front ones being the largest. Each tooth is implanted 

 by a simple base in a shallow cup-shaped socket, with a 

 slightly raised border, to which the circumference of the 

 tooth becomes anchylosecl. The tooth is loosened by absorption 

 and shed to make way for a successor. These are developed 

 on the inner, hind, and fore part of the base of the old tooth. 

 The teeth are usually shed alternately. They consist of 

 osteodentine, dentine, and cement. The first substance occu- 

 pies the centre ; the last covers the superficies of the tooth, 

 but is introduced into its substance by many concentric folds 

 extending along the basal half. These folds are indicated by 

 fine longitudinal, straight stria? along that half of the crown. 

 The section of the tooth at that part (see fig. 65, tooth-section) 

 gives the same structure which is shown by a like section of 

 a tooth of the Lcpidosteus oxyurus.* 



The same principle of dental structure is exemplified in 



* Wvman, American Journal of the Natural Sciences, Oct. 1843. 

 N 



