AND A DENTAL NOTATION. 
495 
determine the homotypal teeth in those two jaws with the same certainty as the 
homologous teeth in the jaws of different species. 
When the mouth is shut the teeth in the lower jaw are a little in advance of their 
homotypes in the upper jaw; thus in the Carnivora the great canine tooth of the 
lower jaw always passes in front of the canine above. Even in the human subject 
this characteristic relative position is shown by the molars and premolars, when the 
upper incisors are produced beyond the lower ones. 
The existing species of Mammalia that retain the typical formula of dentition, viz. 
.3-3 1-1 4-4 
* 3-3’ ^ 1-r -^4-4’ 
m 
3-3 
3-3 
44, 
are few: but that formula was much less frequently departed from in the species of 
placental Mammalia which were first introduced into this planet. This is a very sig- 
nificant fact, and became manifest in the course of working out such typical formula 
by tracing and comparing the development of the teeth in the recent species. 
In the oldest known strictly carnivorous mammal, e. g. the Hycenodon, remains of 
which have been discovered in the newer eocene deposits of Hampshire, and in the 
miocene formations of France, the complete typical dentition is retained, and each of 
the three true molars presents the peculiar trenchant form of crown which charac- 
terizes the single tooth called by Cuvier ‘ le dent carnassi^re ’ in the Lion : here, 
therefore, we use the term ^ molar ’ in the same technical or arbitrary sense as the 
term ‘ incisor ’ when applied to the tusks of the Elephant or the prongs of the Hip- 
popotamus. In the mixed-feeding Amphicyon, a larger extinct miocene Carnivore? 
allied to the Plantigrades, the three true molars have broad tuberculate crowns. 
Almost all the herbivorous genera of the eocene and miocene tertiary periods had 
the typical number and kinds of teeth, as, e. g*., Anoploihermm, Paloeotherium, Dicho- 
don, Cheer opotamus, Dichohune, Anthracoiherium, Hyopotamus, Hyracotherium, Oplo- 
therium, Merycopotamus, Hippohyus, &c. When a modern genus or family has been 
represented as far back as the miocene period by extinct species, it is usual to find 
some nearer approach made by such species to the typical dentition than is made by 
the existing species. Thus, in existing Ruminants, the first premolar is suppressed ; 
but in the ancient Dorcatherium it was retained. In the modern Hippopotami the 
incisors are reduced to and the first premolar is speedily lost; in the oldest known 
representatives of the genus — the Hexaprotodon of the Himalayan tertiary beds — the 
incisors were in the typical number as the name imposed upon it by its disco- 
verers, Cautley and Falconer, indicates, and the first premolar was long retained ; 
the whole dentition, in short, presented the typical formula. • 
The existing species of the gigantic Proboscidian family, viz, the Asiatic and African 
Elephants, are totally devoid of incisors in the lower jaw, and all their grinding-teeth 
succeed each other horizontally ; so that it is only by a more than proportional in- 
