Ichthyosauria. 
35 
In some of the Ichthyosaurs the jaws are prolonged into a 
long and slender rostrum ; others have short and robust heads, 
and jaws armed with large teeth. A most perfect example of 
the long and slender- jawed form of Ichthyosaurus tenuirostris , 
from the Lower Lias of Street, Somerset, was presented in 1884, 
by Alfred Grille tt, Esq., of Street, Somerset. 
Two other genera slightly modified from Ichthyosaurus, 
namely Baptanodon and Ophthalmosaurus, are included with it 
here. 
Some diversity of opinion exists as to the homology of the 
three bones which articulate with the distal extremity of the 
humerus and femur in the two latter forms. Marsh and Hulke 
correlate them as in Ichthyosaurus , with the radius, intermedium, 
and ulna ; Seeley terms them radius, ulna, and olecranon; whilst 
Baur considers that they represent the radius, ulna, and 
pisiform. 
a b c 
Fig. 44. — (a) Lateral and (b) profile views of a tooth of Ichthyosaurus platyodon (Cony- 
beare) Lower Lias, Lyme Regis, Dorsetshire, (c) Tooth of Ichthyosaurus communis 
(Conybeare), Lower Lias, Lyme Regis, Dorset. 
It has been almost certainly shown by Baur that the 
Ichthyopterygia have taken their origin from terrestrial or 
amphibious ancestors. The structure of the limb in the more 
generalized species of Ichthyosaurus indicates that the pectoral 
limb consists primarily of only four digits, the first digit being un- 
represented, and the fourth and fifth arising in the usual manner 
from the ulnare. The additional rows of phalangeals in the 
more specialized forms it is suggested are due to a splitting up 
of the radial (2nd) and intermedial (3rd) digits, the presence of 
Gallery, 
No. 11. 
Wall-case, 
No. 13. 
Table-case, 
No. 14. 
