INVERTEBRATA. 453 



horny beak. The skull is globular with indistinct sutures, 

 and the lower jaw is articulated to a quadrate bone as in 

 reptiles. The pectoral girdle consists of scapula, strong 

 coracoid articulating with the sternum, and a clavicle 

 united with that of the other side. The sternum is broad 

 and long and produced ventrally, except in ostriches and 

 their allies, into a deep keel. The sacrum consists of 

 about twelve vertebrae united together, the ilium is long 

 and parallel to the sacrum, neither pubis nor ischium meet 

 in a ventral symphysis. There is only one aortic arch, the 

 right, and the oxygenated blood and deoxygenated blood 

 are completely separated. In spite of these peculiarities 

 birds are in many respects, as in the possession of a quad- 

 rate, of large coracoids, of an undivided cloaca, similar to 

 reptiles, and therefore the two classes are often grouped 

 together under the name Sauropsida. 



CLASS : MAMMALIA. 



A list of the mammalian characters is given on p. 133 of 

 the original text-book, but the list requires the following 

 additions and explanations. The chief external peculi- 

 arities are the hair, the mammary glands, and the external 

 ear. A hair is a cylindrical rod of epidermic cells produced 

 from the surface of a papilla which lies at the bottom of a 

 follicle sunk in the skin ; the dermal vascular tissue of the 

 papilla does not extend up the centre of the hair as in 

 the ease of a feather. Mammary glands are special en- 

 largements of the sebaceous glands which open into the 

 hair follicles. The external ear is an outgrowth of the skin 

 around the external auditory meatus supported by cartilage 

 and supplied with muscles. The diaphragm dividing the 

 thoracic cavity from the abdominal and the presence of 

 only one aortic arch, viz. the left, are the special features 

 of the circulatory and respiratory systems. 



Of the skeletal characters, epiphyses to the vertebral 

 centra are wanting in Monotremata, some of the Sloths 

 have eight or nine cervical vertebrae, and one has only six ; 

 the Manatee also has only six ; all other Mammals have 

 seven. Coracoids are absent as separate bones, and only 

 represented by the coracoid processes of the scapulae ; in 



