462 CHORDATA, 
found in the 4vsectzvora. In both jaws there arises posterior to the pro- 
tocone (or protoconid) a fourth cone called the Aypocone (or hypoconid). 
All four become regularly arranged, giving a quadruple row of trap-like 
tubercles. This type is called the guadritubercular, often complicated 
by further smaller tubercles forming a mudtdtubercular arrangement. 
The quadritubercular condition is well seen in the hedgehog. 
In both the other series a hypocone also arises and in the lower 
jaw it may be double. 
Thus a quadritubercular condition is produced in the bunodont 
series by a similar development of a hypocone and hypoconid. The 
hypoconid is developed in such a position that il moves up into the 
depression in the primary triangle of the upper jaw, whereas the hypo- 
cone is, like the protocone, between the primary triangles of the lower 
jaw. The consequence is that, when a lateral motion is given to the 
lower jaw and transverse columns are formed by fusion across of the 
tubercles, the upper jaw has a normal fusion of protocone with paracone, 
and metacone with hypocone, but the lower jaw has a fusion of proto- 
conid and metaconid to form the anterior transverse ridge, whilst the 
posterior is formed by the hypoconid and subsidiary cones, the para- 
conid disappearing altogether. Thus is produced the dzlophodont type 
with two transverse ridges, those of the upper jaw alternating with 
those of the lower, This important type is found in kangaroos and 
in tapirs and forms the starting point of the perissodactyle series. 
Further differentiation of the grinding molars is in the direction of 
complex foldings which tend to increase the number and extent of 
enamel-ridges. (See Horse and Ox.) 
Summary.—In the cheek-teeth of mammalia we can distinguish 
the following series :— 
1. Haplociont—single series of simple conical teeth (Odoztoce/z). 
2. Triconodont—single series of teeth with three cusps or cones 
(Triassic Jetatheria). 
3. Tritubercular—series with three cusps, usually with two in 
different position from the other, the whole forming a triple series 
(Triassic Metatheria). 
4- Quadri- and multitubercular—series with four or more cusps 
forming four or more series and retaining typical (insectivorous) char- 
acters (/usectivora). 
5. Secodont series, with cusps united by sharp ridges and often 
increased in number—carnivorous (Carnivora). 
6. Bunodont series, with cusps separated and often increased in 
number, blunt and crushing—omnivorous or herbivorous. (Szde, 
Urside, Primates. ) 
7. Bilophodont and other types, increase of tubercles, transverse 
and longitudinal ridges formed by fusion, complex folding and the 
crowns worn flat during life—herbivorous (Ungulata, Nodentia). 
Brain AND NERVOUS SystEM.—The characters dis- 
tinguishing the brain of mammals from that of the other 
Vertebrata are not so striking as one would perhaps be led 
to assume, considering that mammals largely owe their 
