2li 



VERTEBRATE SKELETON 



The oldest Proboscidians {Mcerotherium and Palceomastodoth had incisors in 

 both jaws. From these two lines developed. Dinotherium lacks the upper 

 incisors, the lower turned downwards. True elephants have lost the lower 

 incisors, the upper forming the tusks which reached their extreme in the 

 mammoths. 



SiRENiA. — Although often placed near the whales, the manatees differ widely 

 from them in crania and are more like the Ungulates. The cranial cavity is 

 cylindrical, the foramen magnum large and the zygoma complete, largely formed 

 by the process of the squamosal, the zygomatic bone forming most of the floor 

 of the large orbit, which is separated in part from the temporal fossa by post- 

 orbital processes of frontal and zygomatic, the separation being most nearly 

 complete in Manatus senegalensis (fig. 230). The four occipitalia are separate, 



Fig. 230. 



-Skull of Manatus senegalensis. /, frontal; m, maxilla; o, occipital; p, 

 parietal; prn, prema.xilla; /, tympanic; z, zygomatic. 



the supraoccipital extending forwards into the angle between the fused parietals. 

 The nares are dorsal and the nasals (lacking in Halicore) are small; the greatly 

 elongate premaxillae diverge behind, form the lateral borders of the nares and 

 reach the frontals (in some fossils the nasals cover part of the nasal passages). 

 The short maxilla joins the zygomatic by a lateral process, its palatal processs 

 forming much of the hard palate. The small, often imperforate, lacrimal is 

 absent in some. 



The alisphenoid is without foramina, the lower rami of nerve V passing out 

 through the foramen lacerum; the optic foramen is a long tube in the orbito- 

 sphenoid. The tympanic (a half ring in Halicore, nearly complete in Manatus) 

 is partly fused with the petrosal, between which and the occipitals and squamosal 

 is a gap, largest in Halicore. The palatine part of the hard palate is short. 

 The incisive foramen, between maxilla, and premaxilla, leads to a long tube which 

 passes back to the nares. The lower jaw has a long symphysis, a rather high 

 and small coronoid process and a large, nearly flat condyle. 



