CHAP. Il] THE MAMMALIA 19 



In returning to the subject of the Classification of Mammals 1 , 

 we must then first consider the combination of morphological 

 characters which entitle the animal possessing them to be ranked 

 as a mammal. In view of what has just been noted as regards 

 the different systems, it will not be a matter of surprise that the 

 characters selected are taken from several, and not all from any 

 one system. 



The Distinctive Characters, and the Classification of Mammals 2 . 



The Mammalia are air-breathing vertebrates, with warm blood, 

 and with an epidermal covering in the form of hairs. 



I. The bodies of the vertebrae are in nearly all mammals 

 ossified each from three independent centres, one of which develops 

 into the centrum proper, while the others give rise to two discs of 

 bone — the epiphyses. Also characteristic of the spinal column of 

 mammals are the discs of fibro-cartilage, termed intervertebral 

 discs, which intervene between successive centra. 



II. The skull has two condyles for connection with the atlas, 

 instead of the single condyle of the Sauropsida (Birds and Reptiles) ; 

 and the lower jaw articulates with the skull in the squamosal region 

 without the intermediation of the separate quadrate element 

 always present in that position in Birds and Reptiles 3 . 



1 Notice that the character, viz. the nutrition of the young by the secretion of 

 milk-glands, which has given the Class its name, is not a primarily morphological, 

 but a physiological feature. Its morphological counterpart is found in the mammary 

 glands themselves. The first employment of the term "Mammalia " is ascribed by 

 Gill and Gregory to Linnaeus. 



2 Owing to the courtesy of authors and publishers, the classification of the 

 Mammalia as given by Messrs Parker and Haswell in their Textbook of Zoology, 

 Vol. ii. (Macmillan), is here available ; though it has been followed generally, some 

 modifications necessitated by the advance of knowledge have been made. 



3 Spurrell (P. Z. S. 1906, n. p. 7) points out an interesting feature related to 

 the possession of a large quadrate bone. It is submitted that in one type of mouth, 

 the articulation of the mandible is in the plane in which the teeth meet. This type 

 occurs even among the Mammalia, e.g. the Carnivora. Lateral movements are 

 negligible in this type. In the second type, the articulation is not in the same 

 plane as that in which the teeth meet. In Keptiles it is below that plane, and the 

 displacement is due to the length of the quadrate bone. In Mammals the plane of 

 teeth is displaced downwards and this is due to the development of a distinct 

 " ascending ramus " of the lower jaw. In the second type, all the teeth may meet 

 simultaneously, and in many Mammals, lateral movements are possible. 



2—2 



