124 COMPARATIVE DENTAL ANATOMY 



all suppressed except the tusk incisor in the left 

 intermaxillary bone. This grows until it some- 

 times attains the length of eight or nine feet, and 

 a basal diameter of four inches. The exterior of 

 the tusk is marked by spiral ridges, which wind 

 around the tooth from the left. The right side 

 has a small suppressed tusk, but it is like the con- 

 cealed tusk of the female, for the male only has 

 the long tusk. The porpoise has eighty to ninety 

 teeth of flattened cusped form, which alternate 

 like the teeth of reptiles, with the largest teeth 

 on the side of the jaws. The Dolphin has the 

 most numerous teeth of any mammal, often reach- 

 ing one hundred and ninety to two hundred, as in 

 the common dolphin. These teeth are sharp, 

 slender, conical, and recurved ; diminishing in size 

 each way from a long tooth on the side of each 

 jaw. The teeth of the Cetacea exhibit an inter- 

 esting degradation of form, as tooth germs are 

 found in fetal life in the edentulous species. The 

 gradual closing and obliteration of the dental 

 groove in fossil forms has even been traced. The 

 Sirenia comprise the herbivorous Cetaceans, of 

 which there are but two genera now living, — the 

 Dugongs and Manatees. In the Dugong (Fig. 40) 

 the end of the upper jaw is bent downward, form- 

 ing an angle with the body of the jaw, in the end 

 of which two tusk-like incisors are implanted. 

 These are thickly coated with enamel on one side, 



