GLOSSARY 669 



Marsupialia, Opossums, Kangaroos, etc., etc. ; only order of the infraclass 

 Didelphia. 



Marsupium, the hairy pouch in which the young Marsupials are carried. 



Masseter muscle, a muscle of mastication, attached to the lower jaw and in- 

 ferior border of the zygomatic arch. 



Mastoid, that part of the periotic bone which is exposed on the surface of the 

 skull. 



Mastoid process, a spine-like outgrowth of the mastoid. 



Maxillary, the upper jawbone. 



Medullary cavity, the marrow cavity of a long bone. 



Mesozoic, the middle era of geological time. 



Metacarpal, a member of the metacarpus. 



Metacarpus, the long bones of the manus, or fore foot. 



Metapodial, a metacarpal or metatarsal. 



Metatarsal, a member of the metatarsus. 



Metatarsus, the long bones of the pes, or hind foot. 



Miocene, the fourth of the Tertiary epochs. 



Monodelphia, placental manmials ; the higher infraclass of the Eutheria. 



Monophyletic, derived from a single line of ancestry. 



Monotremata, Duck-billed Mole and Spiny Anteaters ; the only existing order 

 of the Prototheria. 



Monte Hermoso stage, upper Pliocene of Argentina. 



Mouse Deer, chevrotains ; suborder Tragulina. 



Mystacoceti, Whalebone Whales ; order of the Cetacea. 



Nasal, one of a pair of bones, forming the roof of the nasal passage. 



Navicular, central bone of the tarsus. 



Neural arch, the bony arch of a vertebra. 



Neural canal, the cavity in the arch, lodging the spinal cord. 



Neural spine, or spinous process, the projection arising from the summit of 



the neural arch. 

 Notostylops Beds, see Casa Mayor stage. 



Occipital condyles, a pair of knob-like protuberances from the occiput for 



articulation with the first vertebra. 

 Occipital crest, an elevated bony ridge around the margin of the occiput. 

 Occiput, the posterior surface of the skull. 

 Odontoceti, Toothed Whales ; order of Cetacea. 

 Odontoid process, a peg-like projection from the body of the second vertebra, 



which fits into the ring of the first. 

 Olecranon, the heavy projection from the upper end of the ulna, forming 



the point of the elbow. 

 Oligocene, the third of the Tertiary epochs. 

 Opposable, used of the thumb and great toe, when they can be opposed to the 



other digits. 



