100 



MEMOIRS OF THE I'NIVERSITY OF CALIFORNIA. 



in some instances been buried almost entire before decomposition could permit 

 a scattering of the elements. The figures of an anterior and a posterior limb 

 shown by Repossi (1902, tav. 9, figs. 2 and 3) evidently represent almost perfect 

 extremities. Contrary to what might have been expected from what is known 

 of some other primitive Triassic ichthyosaurians, the posterior limbs are very 

 considerably reduced in size below the dimensions of the anterior limbs. Evi- 

 dently the hind limbs had already to some extent lost their usefulness, and 

 were degenerating. 



The anterior limb while considerably larger than the posterior one is also 

 in some respects more specialized. In other words it has retained larger size 

 as advantageous in the performance of its peculiar functions connected with 



locomotion by swimming. The hind limbs not being re- 

 quired in swimming as much as the fore limbs have grad- 

 ually become reduced, but have not in the course of this 

 reduction been forced to acquire as many new characters in 

 adaptation to aquatic conditions. The anterior limb (fig. 

 127) is more primitive than that of the typical TclitJii/o- 

 MiHnts in several important particulars: the epipodial ele- 

 ments are elongated and separated by a wide gap; the 

 number of digits is the normal five; the phalangeal ele- 

 ments are less numerous than in most ichthyosaurs ; in 

 many cases the phalanges are relatively elongated and show 

 a median constriction. The humerus of Mixosaurus does 

 not show particularly primitive characters. Compared with 

 most species of Ichthyosaurus it is a relatively short and 

 broad element, The humerus figured by Repossi (1902, tav. 

 9, fig. 2), with the remainder of the limb, though a relative- 

 ly wide bone is verv much narrower than in certain other 

 specimens in the Milan collection. In one specimen the 

 width of the Immerus equals more than two-thirds of the 

 length. On the whole, the shortening of the humerus in 

 Mixosaurus is fully as pronounced as in Ichthyosaurus or B<i}>t<nio(lou. In 

 some cases it is almost as extreme as in some of the specialized ichthyosaurs of 

 the shastasaur group. In one specimen showing what is apparently the lower 

 side of the humerus a well developed pectoral ridge extends along about two- 

 thirds of the length of the bone. 



In Mixosaurus hyperdactyly seems not to have developed, and hyperphalangy 

 is not as far advanced as in Ichthyosaurus. The number of the phalanges 

 is nevertheless far in advance of the normal, at least ten separate elements 

 being present in the median digits. More strikingly primitive than the other 

 characters of the digits is the tendency of particularly the proximal three to 



Fig. 127. Mixosaurus cor- 

 iialiiinux (Bassani). An- 

 terior limb,X%. (Adapt- 

 ed from Eepossi.) 



