172 



COMPARATIVE ANATOMY 



Mammals must also be briefly mentioned. In Bats, the phalanges 

 are greatly elongated to support the wing-membrane ; the fore 

 limbs are modified for digging in certain Mammals (e.g. Echidna, 

 Mole) ; and in the Cetacea (cf. p. 170) and Sirenia the digits are 



not free, and serve as supports 

 for the fin-like paddles. Hind 

 limbs are absent in the two last- 

 mentioned orders (cf. p. 155), 

 but indications of them can be 

 seen even externally in very 

 young embryos of the Porpoise. 

 In the leaping Jerboa (Dipus), 

 the metatarsals are much elon- 

 gated, and may even become 

 ankylosed, as in Birds. 



A bony knee-cap or patella,, 

 such as occurs in certain Lizards 

 (e.g. Varanus) and in Birds, is 

 present in most Mammals, being 

 wanting only in Cetacea, Sirenia, 

 Cheiroptera, and some Marsu- 

 pialia. It has no genetic con- 

 nection with the bones of the 

 Fin. 136. -LONGITUDINAL SECTION thigh and shank, and so is in no 



THROUGH THE MANUS OF THE LLAMA i n ., , , , , 



(Anchenia). .(After M. Weber.) Wa 7 comparable with the oleo- 



cranon of the ulna, as was 



'tK^f&J^iibi formerly supposed. It is a true 

 7, cushion composed of elastic con- sesamoid bone, such as occurs in 

 nective tissue (more strongly de- connection with many of the 



veloped in the Elephant, in which j n di V i'Hiifll im'nt^ nf thp Him't^ 

 also the phalanges are more vertical). VMUai joints Ol the digits, 



which has arisen m the tendon 



of the quadriceps femoris muscle in consequence of the friction 

 between this tendon and the condyle of the femur. 



