Ichthyosauria. 



35 



In some of the Iclithyosaurs the jaws are prolonged into a Gallery, 

 long and slender rostrum ; others have short and robust heads, ^^ ^^' 

 and jaws armed with large teeth. A most perfect example of jj-q_ {q^ ' 

 the long and slender-jawed form of Ichthyosaicrus tenuirostris, 

 from the Lower Lias of Street, Somerset, was presented in 1884, 

 by Alfred Gillett, Esq., of Street, Somerset. 



Of Ophthahnosaurus icenicus a fine shoulder-girdle and 

 paddle obtained by Mr. Leeds from the Oxford clay near 

 Peterborough are mounted in Wall-case 14. In this genua 

 and in Baptanodon from N. America three bones articulate 

 with the distal end of the humerus, and some diversity of 

 opinion exists as to their homology. Marsh and Hulke cor- 

 relate them as in Ichthyosaurus, with the radius, inter- 

 medium, and ulna ; Seeley terms them radius, ulna, and 

 olecranon ; whilst Baur considers that they represent the radius, 

 ulna, and pisiform. 



Wall-case, 

 No. 14. 



Fig. 44. — (a) Lateral and (b) profile views of a tooth of Icldhyosaiirus jilatijodon (C0B7- 

 beare); Lower Lias, Lyme Regis, Dorsetshire, (c) Tooth of ^c/i(A^oi-ait/'iis communis 

 (Conybeare) ; Lower Lias, Lyme Regis, Dorset. 



It has been shown by Baur that the Ichthyopterygia 

 have almost certainly 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 uhiare. 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 



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