No. 2.] THE HARD PARTS OF THE MAMMALIA. 143 
BBB. Anterior limbs with flexible joints. Ungual phalanges compressed 
and pointed ! (Unguiculata). 
§. Tarsal and carpal bones generally in linear series. 
e. Teeth without enamel; no incisors. 
Limbs not volant; hemispheres small, smooth; mastica- 
fom orthal «.) s.r eed es...) Qs Egentata. 
ee. Teeth with enamel; incisors present. 
No postglenoid process; mandibular condyle round; 
limbs not volant; hemispheres small, smooth; masti- 
Gition proal. 2. )-. - 2: queen rete . 10. Rodentia. 
Limbs volant; hemispheres small, smooth . , . » Ut. Chivoptera: 
A postglenoid process; mandibular condyle transverse ; 
limbs not volant ; no scapholunar bone?; hemispheres 
small, smooth; mastication orthal . . . 12. Bunotheria. 
A postglenoid process; limbs not volant; with a scapho- 
lunar bone; hemispheres larger, convoluted; masti- 
eauionOrthal. ../ . 3. <aeet eee eee ese (a: 5 E3. Carnauprya: 
88. Tarsal and carpal bones alternating. 
Fore limbs prehensile; mandibular condyle transverse ; 
feeua. with enamel. .- 2) eae ee TA Ansylopada. 
Paleontology has cleared up the phylogeny of most of these 
orders, but some of them remain as yet unexplained. This is 
the case with the Cetacea, the Sirenia, and the Edentata. The 
Marsupialia can be supposed with much probability to have 
come off from the Monotremata, but there is as yet no pal- 
eontological evidence to sustain the hypothesis. No progress 
has been made in unravelling the phylogeny of the Cetacea 
and Sirenia. The results attained by the study of the paleon- 
tology of the other orders may be summarized as follows :— 
First. It is probable that the common ancestors of the pla- 
cental and implacental lines of Mammalia are known to us in 
some of the types of the Jurassic period. Whether they were 
marsupial in the sense of possessing an external pouch for the 
young or not, is immaterial. They were probably marsupial in 
brain characters, in the structure of their reproductive system, 
and in the absence of placenta. To this source the existing 
polyprotodont marsupials may be traced, through such forms as 
Myrmecobius. The Multituberculate type has a cotemporary 
history, and one distinct from that of the Polyprotodontia, and 
its ancestry has not been yet discovered. Their earliest forms 
1 Except Mesonyx and some Rodentia and Edentata. 
2 Except Erinaceus. 
