Macalister — On Cranial Osteology of Sloths. 139 



XVIII. — On a Few Points in the Ceanial Osteology oe Sloths./ 

 By Alex. Macalistee, M.B., Professor of Comparative Anatomy, 

 Dublin University. (With Plate 13.) 



[Read January 25, 1875.] 



In the Museum of the University of Dublin there ai'e several very 

 good specimens of Sloths, some of which were brought home from 

 South America by the Kev. J. M'Gregor Ward, lately Chaplain to 

 H. M. S. Egmont, and some by Dr. Newton. In one of these, a very 

 young specimen of Bradypus gtdaris, measuring only 10 inches in 

 length of body, the skull exhibits several curious points. 



1st. A vertical medial supra-occipital suture, extending from the 

 middle of the back of the foramen magnum to the back of the sagittal 

 suture. Then the supra-occipital consists of two lateral symmetrical 

 pieces. 



2nd. There are three wormian bones, two to the right, and one to 

 the left of the median line in the place of the interparietal bone. 



3rd. The post premaxillary tooth (Caniniform molar of Cholcepus) 

 is very sharp-pointed, directed backwards and inwards. 



4th. Owing to the absence of the frontal sinus (which in the adult 

 B. torquatus occupies the whole of the frontal bone), the skull narrows 

 from the parietal eminences forward. There are two slight fissures, 

 one on each side of the middle of the frontal suture. 



5th. The pterygoids have even so early a trace of the " Arcto- 

 pithecus" thickening. The weak intermaxillaries are very faintly 

 discernible, with a slight boily nucleus. In the accompanying Plate, 

 figure 1 shows the back of the skull with the vertical occipital suture, 

 figures 2 and 4 show the peri-orbital bones, and the relation of the 

 lachrymal to the malar. Figure 3 shows the group of wormian bones. 

 Figure 5 shows the solid stapes resembling the columella of birds, 

 characteristic of the sloths. 



