n4 MEMOIRS OP THE UNIVERSITY OF CALIFORNIA. 



ABDOMINAL RIB PLASTKOX. 



Elements of the abdominal rib plastron are known with nearly all of the 

 Triassic forms, but are in most cases scattered or disturbed to such an extent 



that their arrangement cannot be clearly de- 

 termined. In one specimen of Cymbospon- 

 dylus petritius a portion of the rib plastron 

 shown in fig. 65 was preserved without dis- 

 placement of the elements. Each transverse 

 series consists of a broadly V-shaped median 

 piece and two lateral pieces. The median ele- 

 ment is not divided into two parts as in some 

 forms of Ichthyosaurus, but is produced for- 

 ward as a prominent knob at the point of un- 

 ion of the lateral arms. All of the elements 

 of the plastron seem to be heavier and more 

 robust than in Ichthyosaurus. On this speci- 

 men there seem to be two transverse series of 



. ... Cymbospondym petnnus Leidy. Ab- 

 dominal rib plastron, x % - abdominal ribs for each pair of true ribs. 



In Mixosaurus the nature of the abdominal ribs is, according to Repossi's 

 figures (Repossi, 1902, tav. 8, fig. 1, and tav. 9, fig. 1), much as in Cymbospon- 

 dylus, excepting that the elements are possibly less robust, and the anterior pro- 

 jection of the median pieces is acuminate, and is longer. 



LIMB ARCHES. 



The Triassic Ichthyosauria differ generally from the later types in the closer 

 agreement in size of the corresponding elements in the two limb arches. 

 Though there are some notable exceptions, as in Mixosaurus and Merriamia, 

 the anterior and posterior limbs, and consequently the pectoral and pelvic 

 arches do not differ as widely in size as they are commonly seen to do in 

 Ichthyosaurus and the later genera. A second difference generally distin- 

 guishing the arches of the earlier forms is found in the greater expansion 

 of most of the elements, particularly of the pelvic arch. The large size and 

 more robust character of the elements of the pelvic girdle are to be interpreted 

 as directly indicating greater efficiency of the posterior limbs. The greater 

 strength of the hind limbs probably indicates less effectiveness in the caudal 

 fin. The anterior arch as a whole is heavy, but is not stronger than the arch 

 of some forms of Ichthyosaurus. 



Pelvic Arch. In most cases the elements of the pelvis differ so far from 

 the corresponding bones of post-Triassic forms that they might readily be 



