1917] 



Stock: Skull Structure of Mylodont Sloths 



169 



This splendid series of specimens receives added importance in 

 that it demonstrates completely how the first tooth of the superior 

 series may disappear. In skull 694, a specimen originally of the third 

 group, the alveolus for the first tooth is very well developed. On 

 the left side of the palate the first alveolus was originally as well 

 developed as on the right side. Around the border of the former 

 alveolus an irregular, spongy growth of bone is formed which shows 

 a tendency to grow toward the middle of the tooth-socket. That this 

 actually takes place is well exhibited by no. 642, in which the closure 

 of the first superior alveoli has progressed much further. In this 

 skull the first alveolus of the left side is almost completely closed, 

 while that on the right side has a shallow, crescentic opening along its 

 lingual border. 



No. 693 illustrates how the entire alveolus may be filled by a 

 spongy growth of bone. The final stage is reached in no. 21156, Univ. 

 Calif. Coll. Palae., for example, or in the Nebraska skull described by 

 Brown, where the first superior tooth has entirely disappeared, its 

 former presence being indicated by a scar in front of the second 

 superior tooth. 



It is interesting to note in this connection that somewhat similar 

 observations on the closure of the first superior alveoli in South 

 American mylodonts have been made by Burmeister, 7 who states : 



. . . Der Schadel des hiesigen Museums [a skull referred to Grypotherium and 

 contained in the collections of the Museo Nacional at Buenos Aires] hat noch 

 offene Alveolen im Oberkiefer; in dem von Reinhardt abgebildeten Schadel sind 

 sie schon ganz mit Knochenmasse gefiillt, aber als Narben sichtbar geblieben. 



AVhatever the initial cause of the disappearance of the first 

 superior tooth may have been, it is certain that in some cases the 

 tooth was well developed at the time of its disappearance. It is 

 conceivable that the tooth may have broken off during the life of the 

 individual, an accident which might readily have occurred, consider- 

 ing the habits of the animal. A rapid growth of bone around the 

 periphery of the alveolus may perhaps have prevented the tooth again 

 becoming functional, and as a result resorption took place. It does 

 not seem probable that such resorption was due to old age alone, 

 although this may also have been a factor. In other skulls variation 

 in size of the first tooth favors the idea that growth may in some indi- 

 viduals have been inhibited from the start. In no. 21158 Univ. Calif. 



i Burmeister, H., S.-B. Akad. Berlin, p. 1132, 1886. 



