SKELETON OF MONOTREMATA. 



317 



episternum, ib. t. The sacrum consists of two vertebras in 

 Ornithorhynchus, and of three in Echidna. 



There are thirteen caudal vertebrae in the Echidna, fig. 201. 

 The first is the largest, with broad transverse processes, the rest 

 progressively diminishing, and reduced, in the six last, to the 



201 



^-^5^>^S:~ 



^'^N.^ 



Skeleton of Echidna. Lxxrii. 



central element. The Ornithorhynchus, fig. 199, has twenty-one 

 caudal vertebrae, of w^hich all but the last two have transverse 

 processes, and the first eleven have also spinous and articular 

 processes. The pleurapophysial parts of the transverse processes 

 are distinguishable in half-grown animals. The transverse pro- 

 cesses are broad and depressed ; they gradually increase in length 

 to the tenth caudal, then as gradually diminish to the twentieth ; 

 their extremities are expanded, and, from the fifth backward, are 

 thickened and tuberculate. The spinous processes progressively 

 diminish in height from the first caudal. Hypapophyses are 

 developed from the bodies of the third to the nineteenth caudal 

 vertebra inclusive ; but there are no hiemapophyses articulated to 

 the vertebral interspaces, as in many Marsupials. In the Echidna 

 hypapophyses are absent ; but rudiments of hnemapophyses are 

 connected with the interspaces of one or tAvo of the middle ver- 

 tebrae of the tail. The caudal vertebrae in the Ornithorhynchus 

 are of nearly the same length to the tAvo last ; they progressively 

 diminish in vertical diameter as they recede from the trunk, and 

 are chiefly remarkable for their breadth and flatness ; resembling 

 in this respect the caudal vertebra? of the Beaver and of the 

 Cetacea ; the horizontally extended tail having a similar relation 



