192 EXTINCT, MONSTERS 
rightly considered them to be adapted for swimming, and, with 
his usual foresight, concluded that this monster was a marine 
reptile of great strength and activity, having a large tail flattened 
vertically and capable of being moved from side to side with 
such force and rapidity as to be a powerful organ of propulsion, 
capable of stemming the most agitated waters. The large conical 
recurved teeth, the largest of which was nearly three inches long, 
are well seen in Figs. 67 and 68. Dr. Mantell was fortunate 
| enough to find, in the year 1820, some vertebrae from the English 
Chalk, near Lewes, which were identified as belonging to a 
Mosasaurus. It is now known that this reptile attained the 
length of forty-five feet. 
In 1831 a portion of a lower jaw with large conical teeth was 
discovered in the Chalk near Norwich. But these teeth were 
not quite similar to those of the Maestricht specimen, and 
Sir Richard Owen therefore founded upon them the new genus 
Leiodon.! But Leiodon must have been very similar to Mosa- 
saurus. 
Of late years many fine specimens have been discovered in 
North America, and the labours of Leidy, Marsh, and Cope have 
been of the greatest service in completing our knowledge of this 
strange group of saurians. In the American Cretaceous seas 
they ruled supreme, as their numbers, size, and carnivorous 
habits enabled them easily to vanquish all rivals. Probably 
some of them were seventy-five feet in length, the smallest being 
ten or twelve feet long. In the inland Cretaceous sea from 
which the Rocky Mountains were beginning to emerge, these 
ancient sea-serpents abounded; and many were entombed in 
its muddy deposits. On one occasion, as Professor Marsh rode 
through a valley washed out of this old ocean bed, he observed 
1 Greek—leios, smooth, and odous, tooth. 
