46 



detected among the collection of bones of the Mylodon ; but as the most charac- 

 teristic of these parts, viz. the stylo-hyal bone, is preserved in the collection of 

 the bones of the Megatherium, and also in situ naturali in the petrified skeleton 

 of the Scelidotherium, and are in both these genera modified in adaptation to 

 the articular cavity in the mastoid bone, the presence of the same cavity in the 

 skull of the Mylodon shows that it must have possessed a corresponding stylo- 

 hyal bone. 



In the Megatherium the stylo-hyal has the form of a hammer with a long, 

 slender, shghtly-bent handle, terminated by an obliquely truncated rough surface 

 for syndesmosis with the cerato-hyal. At the opposite end it is subcompressed 

 and expands suddenly in the vertical direction, and terminates posteriorly by a 

 straight but rugged margin ; the upper end of the expansion is thickened, and 

 forms a smooth convexity or head, adapted to the cavity at the under part of 

 the mastoid, and therewith united by a much more secure articulation than that 

 of the lower jaw. The lower end of the expansion is more produced, more 

 rugged, but has an obtuse and rather smooth termination. The inner surface 

 of the hammer-head is impressed by a deep and wide groove. The outer 

 surface has a wide depression at the middle, rough, with several short and well- 

 marked ridges. The length of the specimen described is eight inches, the 

 breadth or depth of the expanded end three inches and a half. In the Scelido- 

 therium the stylo-hyal has the same general form and proportions, but the ex- 

 panded end terminates by a concave line, and has a more bifid character, the 

 upper or articular end being more slender and more produced. 



In the Sloths the hyoid system presents two different conditions. In the 

 three-toed species the body and posterior cornua become anchylosed length- 

 wise with the cerato-hyal or first piece of the anterior cornua, thus forming a 

 single bony arch, to the extremities of which the long stylo-hyal pieces are arti- 

 culated by their anterior ends, and the apparatus is thus divided into three 

 parts. In the two-toed Sloth the original separation of the body and cerato- 

 hyals persists, and the body is characterized by two articular eminences above 

 the angles at which the posterior cornua are sent backwards. 



The preservation of the corresponding part of the hyoid apparatus with the 

 rest of the skeleton of the Mylodon, not only shows its general agreement with 

 the hyoid type in the Sloths, but illustrates the closer affinity of the Mylodon to 



