.MKRKIA.M: THIASSIC K 'I IT! I V( ISAI'IJI A. )<) 



moreover, not fully exposed, and it is not impossible that careful preparation 

 with a needle will show that the apparent gap between the two rib heads is 

 filled by a bridge of bone, now covered by matrix lying in a lateral groove. 

 It is also worth noting that with the form which these ribs appear to have as 

 they lie in the matrix, the upper head could not come into direct articulation 

 with the diapophysis without elevating the shaft above the level of the other ribs. 



Though it is difficult to make certain of the identity of all of the specimens 

 which have been described by Repossi, it has appeared to the writer that the 

 most strongly forked rib figured by him (1902, tav. 9, fig. 10) possibly corre- 

 sponds to a cross-section of a certain neural arch which is associated with other 

 specimens that have evidently been the originals for some of his excellent 

 figures of vertebrae and ribs. 



On the whole, the few instances in which bifurcation of the articular heads 

 of ribs may occur, seem to the writer to be not sufficiently clear to warrant a 

 definite statement that these ribs are normally divided, though a tendency in 

 that direction may possibly exist. On the other hand, that such a disposition 

 to divide should exist is rather remarkable when we consider that the Ameri- 

 can Triassic forms show an evident tendency to lose the bicipital articulation, 

 and that they retain it only in the anterior vertebrae. In the typical Ichthyo- 

 xn/irus and in Baptanodon, where double-headed ribs are well developed 

 through the greater part of the costal series, the pelvic and caudal regions are 

 the only places where they do not normally occur. 



Abdominal ribs are present in the Milan specimens of Mixosaurus. In 

 Repossi 's figures they resemble those of the American Triassic forms in the 

 union of the pieces next the middle line of the abdomen to form a median 

 spine, though the spine appears to be longer and more acute than in Cymbo- 

 spondylus. 



Extremities. As was first shown by Baur (1887, 1), the limbs of Mixo- 

 x< mrus furnish fairly definitely evidence that the ancestral ichthyosaurian type 

 was a shore form. In Baur's figure of the limb the characters of the propo- 

 dial and epipodial segments were somewhat exaggerated, as the bones in this 

 region are really less slender and relatively less elongated. In other portions 

 of the limb, which were possibly not examined by Baur, other characters 

 appear which also point definitely toward an ancestral form with a simple type 

 of limb like that in a shore form. The characters emphasized by Baur are 

 therefore not to be considered as incidental or accidental developments parall- 

 eling what is seen in an ambulatory type of limb, but they express in one way 

 what is also definitely shown in other parts of the limb, or in the limb as a whole, 

 and is distinctly visible in most other parts of the body. 



Fortunately a number of the Mixosaurus specimens in the Milan museum 

 are preserved in a fine shale, and as the individuals are small, the limbs have 



