380 VERTEBRATE ANIMALS OF THE UNITED STATES 



Molars. The hindermost teeth in the mammalian jaw; crushing teeth of certain fishes. 

 Monozygotic. The several offspring which may develop from a single fertilized egg, such as 



identical tnins. 

 Murine. Mouse-like. 



Naked. Without external coverings, such as hair, scales, etc. 

 Nape. The back of the neck. 

 Nares. The nostrils, both those opening on the outer surface of the head and the inner 



ones opening into the mouth cavity. 

 Nasal. Pertaining to the nose or the nostrils. 



Nasal plate. The plate in which the nostril is located in snakes and lizards. 

 Naso-labial groove. A glandular groove connecting the nostril and the lip in certain 



salamanders. 

 Nictitating membrane. The eye-lid at the inner corner of the eye of certain vertebrates, 



which moves across the surface of it. 

 Nocturnal. Being active in the night-time. 



Nose-leaf. A complicated membrane at the end of the snout in certain bats. 

 Notochord. A cylindrical rod which extends the length of the body in chordate embryos 



and forms the basis of the backbone. 

 Nuchal plate. The median anterior marginal plate in the carapace of turtles. 

 Obscure. Indefinite in appearance. 

 Obsolescent. Gradually disappearing. 

 Obsolete. Scarcely visible. 

 Obtuse. Blunt. 



Occipital. Pertaining to the hinder part of the head. 



Occipital condyle. The bony prominence by which the skull articulates with the backbone. 

 Ochraceous. Light brownish yellow in color. 



(Esophagus. The portion of the digestive tract between the pharynx and the stomach. 

 Olivaceous. Yellowish green. 

 Omnivorous. Eating all kinds of food. 

 Opercle. The gill cover; the movable flap on the side of a fish's head which covers the 



gill clefts. 

 Opercular bones. Membrane bones which support the opercle. 

 Opercular flap. The posterior prolongation of the opercle in certain fishes. 

 Operculvim. The opercle. 



Opisthoccelous vertebrae. Those which are concave behind and convex in front. 

 Opisthoglyph snakes. Snakes which have poison fangs in the rear of the mouth. 

 Orbicular. Circular or nearly so. 

 Orbit. The socket of the eyes. 

 Orientation, Organs of. The special sense organs. 

 Oviparous. Egg-laying. 

 Ovoviviparous. Animals producing large eggs which hatch while still in the mother's 



body. 

 Paired fins. The pectoral and ventral fins of a fish. 

 Palate. The roof of the mouth. 



Palatines. Membrane bones supporting the rear portion of the roof of the mouth. 

 Palmate. Spreading; broad and flat, with finger-like projections. 

 Papilla. A minute, fleshy projection. 

 Papillose. Covered with papilhe. 



Parasphenoid. . The membrane bone supporting the roof of the mouth in amphibians. 

 Parotoid. A glandular organ just back of the ear in toads. 



Parietal bones. A pair of membrane bones forming a portion of the top of the skull. 

 Pectinate. Having teeth like a comb. 



