462 CHORD ATA, 



found in the Insectivoya. In both jaws there arises posterior to the pro- 

 tocone (or protoconid) a fourth cone called the hypocone (or hypoconid). 

 All four become regularly arranged, giving a quadruple row of trap-like 

 tubercles. This type is called the quadritiibercular^ often complicated 

 by further smaller tubercles forming a nnUtituhermlar 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 it 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 bilophodont 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 {Odontoceti). 



2. Triconodont — single series of teeth with three cusps or cones 

 (Triassic Met at her ia). 



3. Tritubercular — series with three cusps, usually with two in 

 different position from the other, the whole forming a triple series 

 (Triassic Metatherid). 



4. Quadri- and multitubercular — series with four or more cusps 

 forming four or more series and retaining typical (insectivorous) char- 

 acters {Insectivora). 



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. {Stiidoe, 

 Ursidce, 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, Rodentia). 



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 



