Royal Institution. 335 



forwards, "four," "three," "two," "one." The incisors are counted 

 from the median Une, commonly the foremost part of both upjter and 

 lower jaws, outwards and backwards. The first incisor of the right 

 side is the homotype, transversely, of the contiguous incisor of the left 

 side in the same jaw, and, vertically, of its opposing tooth in the 

 opposite jaw ; and so with regard to the canines, premolars, and 

 molars ; just as the right arm is the homotype of the left arm in its 

 own segment, and also of the right leg of a succeeding segment. It 

 suffices, therefore, to reckon and name the teeth of one side of either 

 jaw in a species, with the typical number and kinds of teeth ; e. y. the 

 first, second and third incisors, — the first, second, third and fourth 

 premolars, — the first, second and third molars; and of one side of 

 both jaws in any case. 



The homologous teeth being thus determinable, they may be 

 severally signified by a symbol as well as by a name. The in- 

 cisors, e. g., by their initial letter ?"., and individually by an added 

 number, i. 1, i. 2, and i. 3 ; the canines by the letter c. ; the pre- 

 molars by the letter p. ; and the molars by the letter m. ; these 

 also being differentiated by added numerals. Thus, the number 

 of these teeth, on each side of both jaws, in any given species, 

 Man e. g., may be expressed by the following brief formula : — 



2 2 1 1 2 2 3 3 



i. ^—z, c. j— -r, p. ^—cy, fn. 2^2 = 32 ; and the homologies of the 



individual teeth, in relation to the typical formula, may be sig- 

 nified by i. 1, i. 2 ; c. ; p. 3, p. 4 ; m. ], m. 2, m. 3 : the sup- 

 pressed teeth being i. 3, p. 1, and j)- 2- 



These symbols are so plain and simple as to form no obstacle to 

 the ready comprehension of the facts explained by means of them. 

 Were those facts described in the ordinary way, by means of the 

 verbal phrases or definitions of the teeth ; as for example, in Man, 

 " the second deciduous molar, representing the fourth deciduous 

 molar in the typical dentition," instead of d. 4, and so on, the de- 

 scriptions of the manifold modifications of the teeth and of dental 

 development must continue to occupy much unnecessary space, and 

 levy such a tax upon the attention and memory, as would inevitably 

 tend to enfeeble the judgement and impair the power of seizing and 

 appreciating the results of the comparison. 



Each year's experience had strengthened the lecturer's conviction 

 that the rapid and successful progress of the knowledge of animal 

 structures, and of the generalizations deducible therefrom, would be 

 mainly influenced by the determination of the homology of parts and 

 organs, and by the concomitant power of condensing the propositions 

 relating to them, and attaching to them signs or symbols equivalent 

 to their single substantive names. In the lecturer's work on the 

 "Archetype of the Skeleton," he had denoted most of the bones by 

 simple numerals : the symbols of the teeth are fewer in number, are 

 easily understood and remembered ; and, if generally adopted, might 

 take the place of names : they would, then, render unnecessary the 

 endless repetition of the verbal definition of the part, harmonise con- 

 flicting synonyms, serve as a universal language, and at the same 



