ORDER ICHTHYOPTERYGIA. 



I 127 



five longitudinal rows of phalangeals in this limb (the marginal rows are 

 wanting in fig. 1031) ; and the radius is transversely elongated (and thus 

 very widely removed from the normal type) with an entire anterior 

 border. It is important to observe that in this group the splitting of 

 the third digit and the two centralia are evidently acquired, and not 

 inherited characters. Like the last, this group may be divided into 

 subgroups. In the typical subgroup, which is confined to the Lias, 

 the teeth (fig. 1032) have their roots strongly fluted, and the dorsal 

 aspect of the humerus has no strongly-marked trochanteric ridge. /. 





g^fe> 



Fig. 1030. — Superior and right lateral aspects of the skull of Ichthyosaurus zetlandicus', from 

 the Upper Lias of Caen. Reduced. Pmx, Premaxilla; Mx, Maxilla; N, Nares ; Na, Nasal ; 

 La, Lachrymal ; Prf, Prefrontal ; Fr, Frontal; Pa, Parietal ; Ptf, Postfrontal ; Sq, Squamosal ; 

 St, Supratemporal ; Po?-, Postorbital ; QuJ, Quadratojugal ; y, Jugal ; A, Orbit; S, Supra- 

 temporal fossa ; Set, Sclerotic plates ; Aid, Mandible; k, Articular; ang, Angular; op, Splenial ; 

 d, Dentary. (After Zittel.) 



intermedins is the species least widely removed from the preceding 

 group ; while the typical /. commimis is distinguished by the extreme 

 width of the paddles. In a second subgroup, which ranges from the 

 Oxford Clay to the Chalk, the roots of the teeth are invested with a 

 thick layer of cement ; the dorsal aspect of the humerus has a pro- 

 minent trochanteric ridge ; and the coracoid differs from that of the 

 preceding subgroup (fig. 1026) by the absence of a posterior notch. This 

 subgroup comprises /. trigonus, and probably other allied forms from 

 the Oxford and Kimeridge Clays ; and also the well-known /. campylo- 

 don, from the Upper Cretaceous of a large portion of Europe, to which 

 /. indiens, from the corresponding strata of India, and perhaps also /. 

 australis, from the Cretaceous of Australia, are probably closely allied. 

 The former is in some respects even more specialised than the follow- 

 ing genera. The species referred to this genus from the Polar regions 

 are considered to be of Triassic age ; and a species from Ceram may 

 be from Cretaceous beds. The genus is unknown in America. 



The most specialised representatives of the family are the genera 

 Opthalmosaitrus, from the Oxford and Kimeridge Clay and probably 

 also the Cretaceous of England, and Baptanodon (Sauranodon), 

 from the Upper Jurassic of North America. In the latter teeth, 

 and even a dental groove, were totally wanting, but in the former 

 small teeth were present. In the former (and probably also the 



vol. 11. Q 



