478 Causes and Course of Organic Evolution 



(8) The vertebrse are procoelous and gastrocentrous. 



(9) The ribs form a true sternum that is calcified and 

 that joins the coracoid. 



(10) The heart consists of two ventricles with incom- 

 plete or complete septum, two auricles, and conus absorbed 

 into the ventricle that has a row of ventricular valves. 



* (11) The right aortic arch only persists and is tri- 

 partite. 



(12) The red blood corpuscles are nucleated, oval, and 

 biconvex. 



(13) The lungs are paired, and have trachea and car- 

 tilages, also bronchus. 



(14) The hypoglossal nerve becomes intracranial, and 

 so posterior to the vagus. 



(15) The kidneys have no nephrostome, but each kid- 

 ney has one ureter. 



* (16) All are strictly oviparous. 



(17) There is an amnion and allantois. 



In Mammalia. 



(1) The skin is smooth, hair-covered, richly glandular, 

 and has abundant nerve endings ; rarely has scales or ossified 

 plates; pigment cells are usually developed. 



(2) The limbs are tetrapodous and four- to five-dactyl, 

 rarely they become rudimentary or absorbed. When 

 digital reduction occurs, digit I disappears first, and later 

 digit V. 



(3) There are embryonic gill clefts, but lungs alone 

 aerate the blood. 



(4) Lateral line sense organs are developed. 



(5) The skull articulates by tw^o exoccipital condyles. 



(6) The ear bones consist of a chain of three or four 

 separate bonelets. 



(7) The mandible consists of one piece, the dentary, 

 that articulates with the squamosal. 



(8) The veretbrfe are flat and gastrocentrous. 



(9) The ribs form a true sternum that is ossified and 

 is free from the coracoid, except in the Monotremata. 



