742 MAMMALIA. 



Reptiles and Amphibians) and a T-shaped prosternum 

 (sometimes called interclavicle), on which the inner ends of 

 the clavicles rest, the outer ends abutting on the acromion 

 of the scapulae. In Ornithorhynchus the ischia form a long 

 ventral symphysis; in Echidna the acetabulum socket for 

 the femur is incompletely ossified (reminding one of Birds, 

 though it is only a secondary peculiarity) ; the pubes bear 

 epipubic bones, as in Marsupials. On the side of the 

 tarsus, in the duckmole, there is a spur perforated by the 

 duct of a gland. This spur is well developed in the males, 

 but rudimentary in the females. The male Echidna has a 

 similar but smaller spur. The fibula 

 has a proximal process like an ole- 

 cranon. 



The brain is smooth in the duck- 

 mole, convoluted in Echidna ; the 

 cerebellum is not covered by the 

 cerebrum, there is a large anterior 

 commissure, and the corpus cal- 

 losum is rudimentary or absent. 

 FIG. 403. Pelvis ot The food canal ends in a cloaca. 



Echidna. The right aur i cu lo-ventricular valve 



\^Ac.? P ^^\ in Ornithorhynchus is partly muscular 



*./., obturator foramen as in Birds, while in other Mammals 



between iscnium and pubis .. j i j r 



(/.). it is membranous and worked by 



papillary muscles attached to it by 



tendon-like cords (chordae tendineae). The temperature 

 of the blood is about 25-28 C, and is noteworthy in 

 being unusually variable. In fact, the Monotremes are 

 imperfectly warm blooded. 



The ureters open, not into the bladder, but into the 

 urogenital canal, as they do in the embryos of higher 

 Mammals. 



The testes remain in the abdomen. The left ovary is 

 larger than the right, as in Birds. The vasa deferentia 

 open separately into the urogenital canal. So in the 

 female do the oviducts, and these have no fringed fim- 

 briated apertures nor distinct uterine region. The penis is 

 attached to the ventral wall of the cloaca, and the uro- 

 genital canal communicates both with the cloaca and with 

 the canal of the penis. The whole structure resembles in 



