18 FOSSIL REPTILES OF DORSET. 



from the neural arches ; they are exceedingly concave, the terminal 

 surfaces nearly meeting at the central hollow ; sometimes the 

 depression commences at once from the border, at others a flat 

 horizontal space intervenes ; the vertebrae of the main-trunk have 

 double-cupped articular surfaces ; and a pair of tubercles to receive 

 the bifurcated rib. It is probable the Ichthyosauri had the power 

 of locomotion on land, resorting occasionally to the shore to deposit 

 their eggs, or to bask upon the rocks. The eye had a peculiar 

 mechanism, being protected by a ring of bony plates sclerotic, a 

 common structure in Birds and Lizards which contracted the area 

 of vision by pressure upon the cornea, enabling it to focus near and 

 distant objects and to sweep in every ray of light when swimming 

 at great depths. Its first appearance in Great Britain is in the 

 Rhsetic beds of Penarth, Glamorganshire. 



ICHTHYOSAURUS COMMUNIS, Conyleare. 



Although the name of this species seems to imply its frequent 

 occurrence, it cannot vie in this respect with Ich. intermedius, 

 which is by far the most common. The skull is very broad 

 behind the orbits, narrowing rapidly towards the snout. The 

 roots of the teeth bulbous, contracting to a conical slightly 

 curved crown, transversely subcircular, with fine longitudinal striae, 

 which become coarser towards the base. The fore-paddles are 

 exceptionally larger than the hind. The number of vertebrae is 

 about a hundred and fifty, of which one hundred at least extend 

 from the pelvis-region to the end of the tail. Found in the 

 Ammonites Bucklandi Zone of the Lower Lias of Lyme Regis, and 

 Charmouth. 



ICHTHYOSAURUS INTERMEDIUS, Conybeare. 



Ich. intermedius differs from the preceding in the smaller size of 

 the orbits, the teeth smaller and longer, the crowns narrower at 

 their base and more acutely conical, the longitudinal striae fewer and 

 the ridges less prominent. The hind-paddles are equally dispro- 

 portionate to the fore-paddles, as in Ich. communis, but narrower in 

 proportion to their length. From the Lower Lias of Lyme Regis, 

 and Charmouth. 



