Characteristics of the Vertebrata. xliii 



body, either persistently or during- foetal life only, as in most of 

 the true Cetacea ; which are always viviparous, and always nourish 

 their young- for longer or shorter periods after Lirtli with the 

 secretion of lacteal glands. The anterior pair of limbs is never 

 wanting ; a perfect diaphragm always exists between the thoracic 

 and abdominal cavities j the aorta is single, and bends over the left 

 bronchus ; the red-blood corpuscles are ' apyrenaematous,^ or ^ non- / 



nucleated/ In all Mammalia, with the exception of the Cetacea 

 and Slrenia, the abdominal vertebrae are separated into a lumbar 

 and a sacral division, by the abutment of the iliac bones upon the 

 vertebrae immediately anterior to the caudal series. In the 

 Marsupial Perameles, however, there may be but one ' sacral ' 

 vertebra, whilst in the Edentata, where the ischium as well as 

 the ilium abuts upon the vertebral column, there may be as many 

 as nine. The cervical vertebrae are, with a few exceptions, neither 

 more nor less than seven in number. Of these, the two first arti- 

 culate with each other and with the two occipital condyles by 

 synovial joints, whilst all the other vertebrae have their centra 

 articulated together by fibro-cartilaginous discs, in the axis of 

 which remnants of the chorda dorsalis are to be found. The 

 number of dorsal vertebrae is very frequently thirteen, but it may 

 vary from ten to twenty-four; that of the lumbar is very fre- 

 quently six or seven, but may vary from two to nine; that of the 

 sacral, as already said, varies from one to nine ; whilst that of the 

 caudal varies from four, as in certain Simiadae, up to forty-six, as 

 in Manis Macrnra. As in Sauropsida, the centres of the vertebrae 

 are always well ossified, but they differ from those of the cold- 

 blooded representatives of that division of the Vertebrate Sub- 

 kingdom, in being always anchylosed with the neural arch in adult 

 life; and from those of both Birds and Reptiles in being during 

 the period of growth provided with epiphyses. There are always 

 two occipital condyles, each of which is constituted by factors from 

 both basi- and ex-occipital. The lower jaw articulates directly with 

 the squamosal element of the cranial walls, the homologue of the 

 OS quadratum of Sauropsida having been withdrawn into the 

 cavity of the middle ear, where it is known as the malleus. The 

 lower jaw itself consists always in adult life of a single bone on 

 each side, which in some Mammals does, and in others does not, 

 anchylose with its fellow of the opposite side at the mental 



