15 



II. AVES. 



Characters. — Aves have the characters of Sauropsida, phis those given 

 in third column of Table at top of previous page. 



STIB-DIVISIONS. 



MAMMALIA. 



Characters. — Mammary glands. Epidermic exoskeleton of hair (mostly). 

 Heart is four-chamhered. Blood warm. Left aortic arch only remains 

 (others present in embryo, but disappear in adult). Coloured blood cor- 

 puscles, circular (oval in camel) and non-nucleated. Skull has two occipital 

 condyles. Lower jaio is simple and articulates directly with the base of the 

 skull {i.e. with the squamosal). Quadrate and hyo-mandibular become 

 internal ear-hones (malleus and incus). Seven cervical vertebra (sloth has 

 nine, manatee six). The vertebrse have terminal epiphyses between the 

 centra. Cerebral hemispheres are united by a corpus callosum. Emlmjo 

 has amnion and allantois and branchial clefts (gill-slits), but never branchim. 

 Teeth are present (absent in spiny ant-eater), and fixed in sockets. 



[The following terras are used in dentition : — Honiodont : teeth all of one sort— 

 e.g. dolphin. Heterodont : having different sorts of teeth— e.g'. dog, rabbit, horse, man, 

 etc. Monophyodont : having one set onlj—e.g. dolphin, sloth. Diphyodont : having 

 two sets of teeth— a temporary (milk) set, and a permanent set— e.g. dog, horse, etc. 

 Upper Inoi.sor : a tooth in the premaxilla. Upper Canine : foremost tooth in the maxilla, 

 provided it be not far from the suture. Premolar : a tooth replacing a milk molar. 

 Lower Canine : a tooth that bites in front of an upper canine.] 



Mammalia are divided into the following twelve orders, arranged under 

 the sub-groups (a), (6), and (c) : — 



(a) Oenithodblphia (Prototheria). — Characters.— Owparows mammalia 

 No placenta. Mammae, but no teats. Goracoids complete. Mar- 

 supial (epipubic) bones. Cerebellum uncovered. No true teeth. 



I. MONOTEEMATA. — The only order of ornithodelph mammalia. 

 Monotremata have the characters of ornithodelphia, plus the following: — A 

 cloaca is present. Goracoids reach the sternum. Glavicles and a T-shaped 

 inter-clavicle. Epicoracoids. Found only in Audralasia. e.g. Ornithor- 

 hynchus (duck-mole) ; Echidna (spiny ant-eater). 



