578 CHORDATA 



eyelids are fused; the mouth is usually extensible; poison fangs are fre- 

 quently present. The ventral surface has scutes. 



41. The Rhynchocephalia resemble the Lacertilia in form, but differ 

 in having a fixed quadrate. 



42. The Crocodilia are elongate, have bony plates in the skin, elongate 

 cloacal opening, fixed quadrate, teeth placed in separate alveoli, and a 

 long swimming tail. 



43. The AVES (birds) are distinguished by feathers, and by the heart 

 completely divided into right and left halves. 



44. Other characters are homoiothermy (warm-blooded), pneuma- 

 ticity of bones, fusion of bones of manus, formation of tibio- tarsus and 

 tarso-metatarsus (intertarsal joint). 



45. The birds are divided into Ratitcr, which lack a furcula and a keel 

 to the sternum, and the Carinalce, in which the sternum is keeled and the 

 clavicles are united to a furcula. To these are added the extinct Saururcs 

 and Odontornithes, with teeth. 



46. The MAMMALIA have a double occipital condyle, hairy skin, and 

 milk glands in the female for the nourishment of the young. Other 

 characters are homoiothermy, the complete separation of the heart, the 

 modification of parts of the visceral arches into the ear bones, high develop- 

 ment of the dentition (formation of roots, usually heterodont and 

 diphyodont). 



47. The mammals are divided into Monotremata, Marsupialia, and 

 Placentalia. 



48. The Monotremata are egg-laying mammals with persistent cloaca; 

 they have a distinct coracoid and an episternum. 



49. The Marsupialia are viviparous, but the young, on account of 

 imperfect nourishment (usually no placenta), are born early and usually 

 carried in a marsupium (marsupial bones). 



50. In the skeleton the inflected angle of the lower jaw is characteristic; 

 uterus and vagina are double. 



51. The Placentalia have well-developed young, nourished in the 

 uterus by a placenta; they have no marsupium nor marsupial bones. 

 The vagina is single (Monodelphia), the uterus simple or paired. 



52. The clawed Edentata and the Cetacea and Sirenia, which have 

 flippers, have a degenerate dentition (teeth monophyodont or lacking). 



53. The hoofed Ungulata (Perissodactyla and Artiodactyla), the 

 Proboscidia, and the clawed Rodentia are herbivorous. 



54. The Chiroptera, which have a flying membrane (patagium), are 

 partly herbivorous, partly insectivorous. 



55. The small Insectivora (with small canines and no carnassial) 



