MKRRIAM: TRIASSIC ICHTHYOSAI IMA. 65 



A se<-ond form is seen in the -rims Mcn-ninn'ii with a n;im>\v paddle, a rela- 

 tively short podial region, three proximal mesopodials, a linear arrangement 

 of the first and second rows of mesopodials, and but three functional digits. 

 In both genera the characters mentioned appear in both anterior and posterior 

 limbs. Of the four other genera recognized in the American Trias, Dc1]>//i- 

 nosanrm, Hli<ixl<is<tin-n*, and Torcioniciinis appear to have a type of limb- 

 structure developed from some form like that seen in Mcrriamia. In Tore- 

 tocneiiHtx, ill spite of the fact that the rib articulation is different from the 

 other three genera, the limb structure is much like that of Merriamia. The 

 American Middle Triassic (7ynibos]>ond//li(s may have a reduced phalangeal 

 formula as in the Upper Triassie genera, and might be presumed to be related 

 to these genera. It shows also a number of characters much like those of 

 Mi.cosaurHs, and may be nearer to that genus than are the American genera 

 from the Upper Trias. 



Mixosaurus and ('yinb(>*]>fid/jlns. Probably the oldest limb elements 

 known in the Trias, consist of some isolated mesopodial or podial elements 

 and a humerus from the Muschelkalk of Germany. The humerus which was 

 described by Fraas (1891, pi. 3, fig. 5) is not materially different from that of 

 Ichthyosaurus. These specimens occur in the same beds with material referred 

 to M. ( ?) atavus and probably belong in one of the forms included in that group. 



Next to the lower Muschelkalk specimens of Europe, the earliest limb 

 material is evidently that of Cymbospondylus (fig. 90) from the American 

 Trias, closely followed in age by Mixosaurus (fig. 91) of the Italian Besano 

 shales. The limb of Mixosaurus is fortunately preserved complete, showing all 

 of the elements in position. In the known specimens of Cymbospondylus the 

 podial region has not thus far been found undisturbed. These two forms 

 represent the most primitive limb structure known in the Ichthyosauria. As 

 the broad type of limb is possibly represented in one and the narrow type in 

 the other, a comparison of the two is necessary in order to determine the 

 common, or primitive characters. 



The anterior and posterior limbs of Cymbospondylus were evidently not 

 greatly different in size, the femur being apparently about one-sixth shorter 

 than the humerus. In Mixosaurus the hind limb is relatively much smaller. 

 The humerus and femur of Cymbospondylus are both relatively slender bones 

 compared with the corresponding elements in other ichthyosaurians. In Mixo- 

 saurus the femur is slender, but the humerus may lack a median constriction, 

 and have a transverse diameter almost equaling its length. In both genera 

 the epipodial elements are relatively long compared with their own transverse 

 diameter, or with the diameter of the proximal mesopodials. The radius is in 

 both forms more slender than in other ichthyosaurian genera, and is much 

 narrowed in the median or shaft region. In Mixosaurus the ulna is convex 



