574 SCIENCE PROGRESS 



the Muschelkalk of Spitzbergen, in which, according to a figure 

 given by Mr. C. Wiman {Bull. Geol. Inst. Upsala, vol. x. 1910), it 

 takes the form of a low falcate fin in the basal third of the tail, 

 continued as a marginal expansion along the whole of the rest of 

 the tail, both above and below. In the young of /. quadriscissus 

 the falcate portion and the marginal fringe are increased in height 

 so as to form a fin approximating to the unsymmetrical or hetero- 

 cercal type of fishes ; with the extremity of the vertebral column 

 bent down into the lower moiety. In the adult of the same species 

 the increase in the size of the fin and the degree of flexure of 

 the vertebral column are intensified, thus producing an approxi- 

 mation to the symmetrical or homocercal type of the fish. A 

 further stage is displayed by /. trigonus posthumus, of the Upper 

 Jurassic Limestone of Solenhofen, in which the fin is almost 

 completely like that of a homocercal fish, while the flexed 

 terminal portion of the vertebral column has diminished in 

 relative proportion, with a marked increase of the angle of 

 flexure. One step more and it would have vanished. A some- 

 what similar grade is presented by the broad-paddled Opthal- 

 mosaurus of the Oxford and Kimeridge Clays. It should be 

 added that in the hind paddle of Mixosaurus the metatarsal and 

 phalangeal bones retain an elongated form and do not form 

 a mosaic-like structure like that found in the later genera, thus 

 indicating a much less wide departure from the ordinary type 

 of reptilian limb. 



Closely connected with the foregoing is an article by Mr. 

 J. C. Merriam (Bull. Dep. Geol. Univ. California, vol. vi. pp. 317- 

 27) on the relationship of the marine saurians from the Trias of 

 Spitzbergen described by Mr. Wiman in the communication 

 cited above. Several of these pertain to the ichthyosaurian 

 genus Mixosaurus and afford important evidence in regard to 

 the structure of the limbs and fins in the members of that genus. 

 On the other hand, Hulke's Ichthyosaurus polaris is made the 

 type of the new genus Pessosaurus, which appears related to 

 Shastasaurus of the American Trias. 



Of greater interest are certain jaw-bones with rows of button- 

 like teeth which have been made the type of the genus Pesso- 

 pteryx. These jaw-fragments appear akin in structure to a 

 specimen from the Middle Trias of Nevada which has been 

 described as Omphalosaurus ; and it is noteworthy that the types 

 of both these genera were accompanied by limb-bones and 



