SKULL MAMMALS 195 



Ornithorhynchus has a separate bone {supra) in the same relative 

 position which later fuses with the orbitosphenoid. 



Peculiar to most Ruminants are the horn-cores on the frontal bones. In 

 the Cavicornia these are permanent and are covered with epidermal horn. In 

 deer there is a pedicel (fig. 205) on each frontal from which, each year, the antlers 

 grow out and are as regularly shed, the antlers consisting of bone developed from 

 the frontal periosteum, covered at first with skin and hair (velvet) which is soon 

 lost. Similar horn cores occur in the extinct Dinocerata. 



The nasal bones which roof the nasal capsules, usually vary in 

 length with that of the snout, but in whales, Proboscidians, Sirenia 

 and a few other forms the nares are far back on the cranium and the 

 nasals are short (nearly vertical in whales, figure 225) and form no 

 part of the nasal roof. The two nasals fuse early in some Old World 

 apes. Rhinoceros and some Insectivores; elsewhere the sagittal suture 

 continues between them. The horns of Rhinoceros, borne on the 

 nasals, are purely dermal and have no horn core. 



The lacrimal lies on the medial (anterior) side of the orbit 

 between frontal and nasal, and usually has both facial and orbital 

 surfaces, but in Primates it is wholly within the orbit. As a rule 

 it is perforated for the lacrimal duct, but not in Elephas, peccary 

 and Sirenia, while it is absent as a discrete element in Monotremes, 

 Manis, seals and Odontocete whales (possibly fused with other 

 bones). In many Ruminants the facial part is excavated for dermal 

 glands. 



The zygomatic bone (lacking in Echidna, some Edentates, isolated 

 Insectivores; reduced in Myrmecophaga, duckbill and rats) arises 

 below the orbit, of which it forms the lateral and lower wall, and 

 sends a process backwards which usually meets the zygomatic proc- 

 ess of the squamosal, forming the zygomatic arch. This arch is 

 incomplete in sloths (fig. 220) in which the connexion is furnished by 

 ligament. Sometimes it has an ascending process which meets the 

 frontal, forming the posterior border of the -orbit, separating it 

 externally from the temporal fossa, but there is usually a connexion 

 between the two beneath this postorbital bar, the gap being closed 

 only in Primates where the alisphenoid intervenes. 



The squamosal bone ossifies from three centres, one for the 

 upper part (squama), one for the zygomatic process, and one in the 

 lower posterior part. The squama is the largest part of the bone, 

 its relative size varying with that of the brain, which, with few 



