Slade.] ^* [March 15, 



la the Dugong, the arch is much less massive. There is no postorbital 

 process from the jugal, aad consequently no separation of the orbital and 

 temporal fossae by a bony orbit. The coronoid process of mandible looks 

 backward. 



Althougli the horizontal curvature of the arch is very slight in both 

 genera of the Sirenia, the temporal fossae are deepened and extended, con- 

 ditions due to the walls of the cranium being compressed in a lateral direc- 

 tion, which materially increases the extent of surface for muscular attach- 

 ment and development. 



In the order Edentata, the jugal arch also offers unusual modifications. 

 In the Myrmecophagidae, it is very incomplete, being composed of the 

 proximal end of the jugal, articulating with the narrow projecting process 

 of the maxilla, and a very rudimentary fragment of the squamosal. These 

 separate portions, however, do not meet, in fact they are widely separated. 

 There is no boundary between the orbital and temporal fossae, the latter 

 being comparatively shallow. The glenoid fossa is a slight cavity running 

 antero-posteriorly, and well adapted to the pointed, backward projecting 

 condyles of the mandible, whose long straight horizontal rami present 

 neither coronoid process nor angle. In Cycloturus, the mandible is some- 

 what arched, and presents a well-marked angular process, as well as a 

 coronoid surface slightly recurved. 



In the Brady podidae, containing the two forms, Bradypus and Choice - 

 pus, the arch is imperfect, consisting of the jugal, which is narrow at its 

 articulation with the lachrymal and maxilla, but which widening out into 

 a broad compressed plate, terminates posteriorly in two processes, the 

 upper pointing backwards and upwards, while the lower looks downwards 

 and backwards. The straight process of the squamosal, although fairly 

 developed, fails to meet either of those of the jugal. There is a post- 

 orbital process of the frontal, which is best marked in Choloepus. The 

 glenoid is shallow and narrow from side to side. The mandible, widest 

 in Choloepus, develops a rounded convex condylar surface, well raised 

 up from the dental series, while the coronoid surface is large and 

 recurved. The rounded angular process projects backwards to a con- 

 siderable extent. The symphysis in both forms is solidified, while in 

 Cholcepus it projects forwards into a spout-like process. The temporal 

 surface for muscular attachment is large, as also are the pterygoid plates. 



In the Dasypodidae, the arch is complete, and in its formation the jugal 

 largely enters. This bone extends from the lachrymal and frontal to the 

 process of the squamosal, the anterior third of which it underlies. There 

 is no postorbital process of the frontal. The glenoid presents a broad, 

 slightly convex, transverse surface. The pterygoids are small. The 

 mandible has a high ascending ramus, the condyle is transverse and above 

 the alveoli, while the coronoid surface is large and the angle broad and 

 projecting. 



In the Manid;ie, the arch is incomplete, owing to the absence of the 

 malar, which if present would occupy almost the exact centre of the arch, 



