SUBCLASSES OF MAMMALIA. 



271 



rim of bone. Besides these general cliaracters of affinity to Birds 

 and Reptiles, there are other striking indications of the same low 

 position in particnlar orders or genera of the subclass. Such, 

 e.g., are the cloaca, convoluted trachea, supernumerary cervical 

 vertebrae and their floating ribs, in the Three-toed Sloth ; the irrita- 

 bility of the muscular fibre, and persistence of contractile power 

 in the Sloths and some other Bruta ; the long, slender, beak-hke 

 edentulous jaws and gizzard of the Anteaters ; the imbricated 

 scales of the equally edentulous Pangolins, Avhich have both 

 gizzard and gastric glands like the proventricular ones in Birds ; 

 the dermal bony armour of the Armadillos like that of loricated 

 Saurians ; the quills of the Porcupine and Hedgehog ; the pro- 

 ventriculus of the Dormouse and Beaver; the prevalence of 

 disproportionate developement of the hind-limbs in the Rodentia ; 

 coupled, in the Jerboa, with confluence of the three chief meta- 

 tarsals into one bone, as in Birds ; the keeled sterniun and wings 



147 



Upper surface 



Brain of Lemur (Stenops tardigradns). LXix- 



of the Bats ; the aptitude of the Cheiroptera, Insectivora, and 

 certain Rodentia to fall, like Reptiles, into a state of torpidity, 

 associated Avith a corresponding faculty of the heart to circulate 

 carbonised or black blood: — these, and the like indications of 

 co-affinity with the Lyencephala to the Oviparous air-breathing 

 Yertebrata, concur with the cerebral character in demonstrating 

 the low position of the Lissencepliala in the Mammalian class. 



The third leadino^ modification of the Mammalian brain is such 

 an increase in the relative size of the cerebrum, fig. 147, Z>, that 

 it extends over half or more of the cerebellum, and of the 

 olfactory lobes. The surface of the hemispheres may be smooth, 

 or with few and simple folds, in the smallest species ; but, as 

 a rule, it is disposed in many gyri or convolutions, fig. 148, 



