ICHTHTOPTERYGIA. O 



In the preeaudal portion of the vertebral column the costal 

 tubercles gradually descend from the summit to the base of the 

 lateral surfaces of the centra ; and since in some forms the superior 

 costal tubercle of the anterior vertebrae is placed partly or entirely 

 on the arch, it will be convenient to call such vertebras or their 

 homologues cervicals, and the succeeding ones dorsals. In the early 

 posterior dorsals the two tubercles may coalesce. 



Some diversity of view obtains as to the homology of the three 

 bones which articulate with the distal extremity of the humerus 

 and femur in the genera Ophthalmosaurus and Baptanodon. By 

 Marsh 1 and Hulke 2 these bones in the pectoral limb are correlated 

 with the radius, intermedium, and ulna ; Seeley 3 , on the other hand, 

 terms them radius, ulna, and olecranon ; while Baur *, whose view 

 is provisionally adopted in the sequel, considers that they represent 

 the radius, ulna, and pisiform. 



It has been almost conclusively shown by Baur 5 that the Ichthyo- 

 pterygia have taken their origin from terrestrial or amphibious 

 animals. The structure of the limbs in the more generalized species 

 of Ichthyosaurus (fig. 32, p. 89) indicates that the pectoral limb 

 primarily consists of only four digits ; the first digit being unrepre- 

 sented, and the fourth and fifth arising in the usual manner from 

 the ulnare. The additional longitudinal rows of phalangeals in the 

 more specialized forms (fig. 23, p. 57) being due to a splitting of 

 the radial (2nd) and intermedial (3rd) digits, the presence of two 

 centralia in the carpus of these higher forms is therefore an acquired 

 and not an inherited character. 



The structure of the palate is essntially the same as in Sphenodon, 

 the pterygoids extending forwards to exclude the palatines (trans- 

 verse bones of Owen 6 and Seeley 7 ) from the middle line. There is 

 a parieto-squamosal (post-temporal) bar. 



In regarding the Ichthyopterygia as nearly related to the 

 Hhynchocephalia, Baur 8 lays stress upon the remarkable resemblance 

 presented by the structure of the pectoral arch, of the abdominal 

 ribs, and of the cranial region to those of Sphenodon. The latter 

 resemblance is exemplified by the similar position of the parietal 

 foramen; the relations of the quadratojugal to the surrounding 

 bones, and especially the separation of its middle portion from tho 



1 Amer. Journ. ser. 3, vol. xix. p. 491 (1880). 



2 Proc. Geol. Soc. for 1883, p. 55. 



3 Quart. Journ. Geol. Soc. vol. xxx. p. 705 (1874). 



4 Bericht xx. Versamml. oberrhein. geol. Verein. p. 18 (1887). 



5 Zool. Anzeig. vol. ix. pp. 245-252 (1886). 



6 See p. 78. 7 See p. 79. 

 8 Amer. Nat. vol. xxi. p. 838 (1887). 



