xm 



PHYLUM CHORDATA 



545 



valent of the protocone, here external, being termed the protoconid, 

 and the others paraconid a'nd metaconid respectively. This 

 trituberculate type of molar is usually complicated by various 

 additions and modifications accessory cusps being added, together 

 with ridges or folds connecting the cusps together. The resulting 

 complex tooth may be modified to act as a cutting (secodont) or a 

 crushing (bunodont) molar. A modification of the bunodont molar 

 is brought about by the cusps, instead of retaining their conical 

 form, being drawn out into the shape of crescents (selenodont). 



The number of the various sets of teeth in the jaws is conveniently 

 expressed by a dental formula, in which the kind of tooth (incisor, 

 canine, pre-molar, molar) is indicated by the initial letter (i., c., p., 

 m.), and the whole formula has the arrangement of four vulgar 

 fractions, in each of which the numerator indicates the teeth of 

 the upper, the denominator those of the lower jaw. Thus : 

 . 3-3 1-1 44 3-3 



or, in a simpler form, since the teeth of the right and left sides 

 are always the same, 



.314 3 

 t. s ,c. 1 ,y. i ,m.g , ==44. 



Echidna has no teeth at any stage. In Ornithorhynchus teeth 

 are present in the young and are functional for a time, but they are 

 thrown off when the animal is 

 about a year old : vestiges of an 

 earlier dentition have been de- 

 tected. The function of teeth is 

 performed in the adult by broad 

 horny plates, one on each upper 

 and one on each lower jaw. 



The Marsupials have the milk- 

 dentition in a degenerate con- 

 dition. Germs of milk-teeth are 

 developed, but with the excep- 

 tion of one the last pre-molar 

 these remain in an imperfect 

 state of development, though 

 they persist, as functionless 

 vestiges, to a comparatively late 

 stage. 



In the adult dentition of the 



TI/T i J/L t ' Fio. 1199. Front view of skull of Koala 



Marsupials the number OI incisors (Phascolarctos cinereus), illustrating dipro- 



in the upper and lower jaws is ^ n e t r . ) and herbivorous d<mtition. (After 

 always dissimilar, except in Phas- 



colomys. With regard to the arrangement of these teeth, the order 

 falls into two series, termed respectively the diprotodont and the 



