1879. | Recent Literature. 447 © 
ome eee alike; that is, that in closing the jaws the external 
usps of the upper series were “brought exactly upon 
a line: ee those of the lower, not over them, as is the case in all 
Herbivora and to some extent in man. This condition he has 
called isognathism. 
2d. That in the Herbivora the jaws were opened and closed in 
mastication with extensive lateral movement of the mandible ; and 
that the teeth were by this greatly modified in respect of ‘their 
tubercular constitution, the tubercles being of the type known as 
crescentic, giving us the long-rooted selenodont type of tooth. 
That in these the width of the jaw, or the distance apart of the 
upper series of molars of opposite sides of the head, greatly 
exceeds the same measurement in the mandible, which results in 
the external or buccal cusps of the upper series closing over and 
external to the buccal cusps of the lower series. This condition 
he has styled anisognathism 
3d. That in some Rodentia and Proboscidea the mandibles were 
moved in a backward and forward, or antero-posterior direction, 
which he has termed the reciprocating movement, with which 
there was also a corresponding tubercular modification, which he 
calls trichecodont—three tubercles forming'transverse ridges, fre- 
quently obsolete, or greatly flattened transverse lamellae, com- 
posed of elongated and united tubercles soldered together by a 
thick cementum layer. (Elephas). Isognathism is usually a 
characteristic of this subdivision, 
n a subdivision including a part of Rodentia and 
Proboscidea (mastodon), the movement of the mandible was both 
reciprocating and lateral, with a still more complex type of denti- 
ion as a result, which he has styled ptychodont—enamel fluted 
and folded upon itself ss Seles and transversely. These 
jaws are usually anisognathou 
These observations, based pele investigations made not only 
upon the skulls but also upon the living animals, afford, he thinks, 
the key whereby to correctly interpret the morphological history 
of the teeth of the higher groups especially when brought to 
bear on the interpretation of the teeth of the great number of 
herbivorous remains which the rocks ts yielded in recent times 
as intermediate or antecedent for 
The number of kinds of a made by the mandibular 
rami is limited by structural impediments, since a bar fixed at one 
en and free at the other, to which the mandible may be com- 
pared, is capable of but a few distirict movements. We find these 
to be essentially those mentioned in the definition of the four 
groups; few others are possible or conceivable. “The first move- 
ment is the vibratory one in a vertical plane; the second is the 
vibratory movement in both the horizontal and vertical planes ; 
the third is the vibration of the rami in an approximately hori- 
zontal plane, neither lateral nor vertical, but an antero-posterior 
