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tlie order Odontornithes, or birds with well defined teeth 
in their upper and lower jaws. One genus is a large carni- 
vorous diver with simple conical teeth imbedded in a groove 
in the dentary bone as in some reptiles. Each tooth pos- 
sesses a crown covered with enamel and a stout conical 
fang. The sternum is without a keel, and the vertebrae are 
of the usual avian type. It was among the largest of 
aquatic birds, being from five to six feet from bill to feet. 
The other genus is Ichthyornis, a creature about the size 
of a pigeon, possessed of a series of ‘simple conical teeth im- 
planted in well defined sockets in upper and lower jaws, of 
a keeled sternum, and of biconcave vertebrae. It w'as of 
aquatic habit, and capable of prolonged flight. The combi- 
nation of a series of conical teeth with a keeUess sternum in 
the former and with biconcave vertebrae in the latter points 
distinctly in the direction of the Keptilia, which stand 
next to the Aves in classification [Sauropsida]. 
5. These Intermediate Forms may he accounted for on 
the Theory of Evolution. 
Before these discoveries were made the existence of inter- 
mediate forms between the Aves and Keptilia had been fully 
recognised. The Deinosauria, the gigantic Iguanodon, 
Megalosaurus and their allies, in the structure of their pelves, 
their sacra, and their hind limbs, were unlike any living 
reptiles, and allied to the birds. On the other hand the 
Archaeopteryx from the Solenhofen slate, a deposit of middle 
oolitic age, presents us S^ith a bird possessing a long tail com- 
posed of separate vertebrae and metacarpals free and disunited, 
characters which are now reptilian and not avian. Thus we 
had proof of the existence of Mesozoic reptiles reaching for- 
ward, so to speak, in the direction of living birds, and fossil 
birds pointing in the direction of the reptilia. In addition to 
this, we now know that certain of the Mesozoic reptiles pos- 
sessed characters peculiar to the living mammalia. Why 
should this be ? Why were the Mesozoic reptilia more richly 
