326 



ANATOMY OF VERTEBRATES. 



confluent, fig. 207, /. The number of phalanges in both Mono- 

 tremes is the same as in other Mammals, viz. two to the thumb 

 and three to each of the fingers. This is not the case in any- 

 Saurian, and the retention of the Mammalian type at the peri- 

 pheral segment of the limb, with the singular deviation from it 

 at the central supporting arch, is not one of the least remarkable 

 points in the osteology of the Monotremes. 



There is a sesamoid bone at the palmar aspect of each of the 

 distal articulations of the phalanges in the Echidna, fig. 207, m, 

 and at all the digital articulations in the Ornithorhynchus, fig. 

 199, H, d. The ungual phalanges are long, depressed, nearly 

 straight, of great strength in the Echidna, in which each of them 

 is perforated at the palmar aspect, fig. 207. 



The pelvis of the Monotremes bears a resemblance to that of 

 Reptiles in the length of time during which the three components 



of each os innominatum remain 

 ^^® distinct, especially in the Echid- 



na ; and in the great develope- 

 ment of the ilio-pectineal spine, 

 which equals in size that of the 

 Tortoise, in the Ornithorhyn- 

 chus : the pelvis of the Echidna 

 resembles that of Birds in the 

 perforation of the acetabulum, 

 fig. 208,^ ; but the pelvis in both 

 Monotremes chiefly resembles 

 that of the higher Lyencephala 

 in the presence of the marsupial 

 bones, ib. e, fig. 199, x. 



The ilium, figs. 199, 208, 62, 

 is a short, strong, trihedral bone, 

 "\vith the upper extremity ex- 

 panded and everted in the Ornithorhynchus : the ischium, ib. 63, 

 has its tuberosity prolonged backward in an obtusely-pointed 

 form: t\iQ pubis in the same animal, besides having the spinous 

 process directed forward, gives off' a second smaller process, which 

 projects outward; this process is present, but less developed, in 

 the Echidna, fig. 208, 64. The pubis and ischium contribute an 

 equal share to the formation of the foramen obturatorium, ib. h, 

 and to the symphysis, d, which closes the pelvis below. 



The marsupial hones, fig. 199, x, x, 208, e, are relatively larger 

 and stronger in the Monotremes than in the ordinary Marsupialia, 

 the Koala excepted ; their base extends along the anterior margin 



Internal view of pelvis, Echidna. 



