232 BRITISH FOSSIL REPTILES. 



This characteristic of the present species is well shown in the group of upper and 

 lower teeth preserved on the right side of the present instructive fragment of the skull, 

 (PI. 2 and PI. 3, fig. 2.) It includes, in an extent of 6 inches and 9 lines, six teeth 

 of the upper jaw ; and, in an extent of 4 inches and a half, four teeth of the lower 

 jaw. Besides the teeth which have preserved nearly their natural positions in respect of 

 each other, there are three or four displaced teeth or fragments of teeth. Of the four 

 teeth of the lower jaw, the three largest, while they have kept nearly their natural 

 position to the teeth above, have slipped out of the groove of the lower jaw during its 

 downward displacement, instead of being separated to the same extent from the upper 

 teeth. In the lower jaw of the Cachalot, where the teeth are lodged in a wide groove, 

 and with the alveoli incompletely developed, they are easily dislodged when decom- 

 position has commenced, and may be stripped away with the firm gum, to which the 

 necks of the teeth adhere more strongly than their fangs do to the rudimental sockets. 



The first of the six teeth of the upper jaw is completely formed, and shows the 

 quadrate root a little compressed in the transverse direction. The rough part of the 

 fang is that which is embraced by the sides of the alveolar grove ; the smooth portion 

 was probably surrounded by a soft slimy gum as far as the enamelled crown. The 

 tooth opposed to this in the lower jaw, and the crown of which passes, as usual, 

 external to it, is a young tooth, with the fang as yet incompletely formed and rounded : 

 its inferiority of size to the tooth above depends on this circumstance. The second 

 tooth above is not so far advanced in growth as the one which precedes or the four 

 that follow it ; the crown and part of the fang of the last, m, of these are broken away, 

 and expose the germ of the young tooth, t, which had penetrated its cavity and was 

 about to displace it. The curve of the rough expanded fangs of the lower teeth is well 

 exhibited in the last two of these teeth. 



The teeth of the Ichthyosaurus campylodon are large in proportion to the slenderness 

 of the elongated jaws, and ofi"er, in this respect, a great contrast with those of the 

 Ichthyosaurus temiirostris : they are even larger in proportion than the teeth of the 

 shorter and thicker jawed Ichthyosaurus communis and Ichthyosaurus lonchiodon, and 

 both the proportions and the form of the teeth determine the specific distinction of 

 the present Ichthyosaurus of the Chalk and Green-sand from any of the known species 

 from the older secondary strata. But there is no modification indicative of a departure 

 from the generic characteristics of the great Fish-lizard : on the contrary, so far as 

 these are manifested by the structure of the jaws, especially the undivided alveolar 

 groove, by the great proportional size of the premaxillaries, and by the thickly cement- 

 covered fangs of the teeth, these characters are rather in excess, and the last of the 

 Ichthyosaurs, far from progressing towards any higher and later form of reptile, might 

 be cited as a type of its peculiar genus. 



