Characteristics of the Vertebrata. xlvii 



The malleus of Mammalia^ tlioug-h limited to auditory functions^ 

 and placed within the cavity of the middle ear^ corresponds to the 

 OS quadratimi , which carries the lower jaw of Sauropsida, being- 

 developed out of the proximal elements of the first visceral arch, 

 whilst the stapes and incus hold a similar relation to the second, 

 and represent the columella of those animals and its supra-stapedial 

 appendage. 



All Mammalia have a urogenital canal independent for a greater 

 or less length, or altogether, of the termination of the intestine ; 

 all male Mammalia have an intromittent organ ; in all female 

 Mammalia during the period of gestation, the blood-vessels of the 

 uterus come into intimate relation with those of the foetus, and 

 provide thus for its nutrition aud respiration during a longer or 

 a shorter portion of its developmental life. 



The reproductive system has furnished a basis for the division of 

 the Class Mammalia into the three Sub-classes, Ornithodelphia, 

 Didelphia, and Monodelphia. 



Sub-class, Ornitliodelpliia. 



The Sub-class, Ornithodelphia, is represented by the single order 

 Mo7iotremata, and the two genera, OrnithorJir/nchis and Echidna. In 

 these animals, as the names Monotremata and Ornithodelj^hia imply, 

 the urogenital and the rectal canals both open by a common cloacal 

 outlet, and the oviduco-uterine ducts remain distinct up to their 

 points of entry into the urogenital canal. In the males, however, 

 there is a perforated penis, which, though not continuous at its 

 base with the urogenital canal, can be brought into apposition 

 temporarily with the orifices of the vasa deferentia, so as to form a 

 functionally distinct sexual canal. The mammary glands have no 

 nipples ; in Echidna, the lacteal ducts open into a pouch-like invo- 

 lution of the integument ; in the Ornithorhynchns they open upon 

 a plane surface ; in both, the embryoes are extruded from the 

 uterine ca\dties whilst in an exceedingly immature state. In the 

 Monotremata, as in Sauropsida, the coracoid reaches the sternum ; 

 they possess an interclavicle ; and the so-called ' marsupial ' bones, 

 which are ossifications of cartilages segmented ofi" from the pubic 

 elements of the pelvis, and w^hich give insertion to a portion of the 

 tendon of the external oblique muscle. The Echidna is edentulous ; 

 the Ornithorhi/nchus has horny plates in the place of teeth. 



