378 



MONOTREMATA. 



supporting the innermost digit or pollex, the 

 trapezoides (g), the index, the os magnum (f), 

 which is almost the smallest, sustaining the 

 medius, and the unciforme (e) the two outer 

 digits: this description is taken from the 

 Echidna: the only essential difference obser- 

 vable in the Ornithorhynchus is the contribu- 

 tion by the os magnum of a greater share to 

 the articulation with the ring-finger. 



In the Echidna all the bones of the fore- 

 extremity are relatively larger and stronger 

 than in the Ornithorhynchus, but this dif- 

 ference is especially remarkable in the meta- 

 carpal bones and two first rows of phalanges 

 Jig. 176, h, i, /c), which are singularly short, 

 broad, and thick. The palm is strengthened 

 by two large sesamoids developed in the flexor 

 tendons in the Echidna; these are sometimes 

 confluent (Jig. 175, /). The number of pha- 

 langes in both Monotremes 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 peripheral 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. 1 75), and at all 

 the digital articulations in the Ornithorhynchus 

 (Jig. 173, u, d). 



Fig. 176. 



in the Ornithorhynchus ; the pelvis of the 

 Echidna resembles that of Birds in the perfo- 

 ration of the acetabulum (Jig. 177, g), but the 

 pelvis in both Monotremes chiefly resembles 

 that of the higher implacental Mammalia in 

 the presence of the marsupial bones. 



Fig. 177. 



Bonet of the fore-foot , Echidna hystrix. 

 ( Cuvier.) 



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 (Jig. 175). 



Of the pelvic extremities. 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 Echidna; and 

 in the great development of the ilio-pectineal 

 spine, which equals in size that of the tortoise, 



Internal view of pelvis, Echidna setosa. 

 ( Original. J 



The ilium (Jig. 177, a) is a short, strong, 

 trihedral bone, with the upper extremity ex- 

 panded and everted in the Ornithorhynchus. 

 The ischium (6) has its tuberosity prolonged 

 backwards in an obtusely-pointed form in the 

 Ornithorhynchus. The pulls in the same 

 animal, besides having the spinous process 

 directed forwards, gives off a second smaller 

 process, which projects outwards; this process 

 is present, but less developed in the Echidna 

 (Jig. 177, f). The pubis (c) and ischium con- 

 tribute an equal share to the formation of the 

 foramen obturatorium (A) and to the symphysis 

 which closes the pelvis below: the symphysis is 

 relatively shorter in the Echidna (<T) than in the 

 Ornithorhynchus. 



The marsupial bones (Jig. 173, A, x x, 177, e) 

 are relatively larger and stronger in the Mono- 

 tromes than in the ordinary Marsupialia, the 

 Koala excepted ; their base extends along the 

 anterior margin of the pubis from the symphysis 

 outwards to that of the spinous process (fig. 

 177, f); they are relatively longer in the 

 Echidna (e) than in the Ornithorhynchus; they 

 always remain moveably articulated with the 

 brim of the pelvis. 



The femur is short, broad, and flattened ; its 

 head rises, like that of thehumerus, from the 

 middle of a broad expanded proximal end, 

 having on each side a strong process, the outer 

 one representing the great, the inner one the 

 small trochanter. In the Echidna a project- 

 ing ridge extends from the great or outer tro- 

 chanter beyond the middle of the bone ; the 

 whole of the inner part of the shaft is bounded 

 by a trenchant edge ; both outer and inner 

 margins of the bone are trenchant in the 

 Ornithorhynchus. The distal end of the femur 

 is expanded transversely, but compressed from 

 before backwards. The rotular trochlea is flat 



