200 PALEONTOLOGY. 



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

 anchylosed. 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. Alternate 

 teeth are usually shed together. They consist of osteodentine, 

 dentine, and cement. The first substance occupies 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 strise along that half of the crown. The section of 

 the tooth at that part (see fig. 84, tooth-section) gives the same 

 structure which is shewn by a like section of a tooth of the 

 Lepidosteus oxyurus* 



The same principle of dental structure is exemplified in 

 the teeth of most of the ganoid fishes of the carboniferous and 

 Devonian systems, and is carried out to a great and beautiful 

 degree of complication in the " old red" Dendrodonts. 



The repetition of this structure in the teeth of one of the 

 earliest genera of Air-breathers, associated with the defect of 

 ossification of the endo-skeleton and the excess of ossifica- 

 tion in the exo-skeleton of the head and nape, instructively 



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



