52 



MEMOIRS OF THE UNIVERSITY OF CALIFORNIA. 



Excepting Torcfoniemus, some of the M.( ?) atai'nx specimens, and a doubt- 

 ful division of the posterior dorsal and anterior caudal rib heads in Mixomurus, 

 double-headed articulation in the Triassic forms is limited to the most anterior 

 region of the column, and in this region it becomes less pronounced in passing 

 back from the most anterior cervicals. In the region of the thorax, where 

 the ribs are long and are particularly influenced by the movements of the lungs, 

 they have a single head. The Ichthyosauria being purely aquatic', high-sea 

 animals may have remained under water for a considerable period at times, 

 and have required a large intake of air before submergence, and full inspir- 

 ation on returning to the surface. In the Cetacea, living under somewhat 

 similar conditions, we find the most specialized forms showing a tendency to 

 reduce the rib articulation of the dorsal region to a single head. This head 

 is in the cetaceans, as in the shastasurian ichthyosaurs, the upper head or the 

 one allowing the widest excursion of the ribs in inspiration. In the Cetacea 

 it may be noticed also that the reduction of the lower rib heads takes place 

 progressively forward, or occurs last in the most anterior ribs, as in Shctsla- 

 saurus. In the shastasaurs a number of distinctly double-headed ribs have 

 remained in the anterior region of the column. In the whales there being 

 no cervical ribs the double articulation may show an apparently more extreme 

 reduction. 



In the Cetacea where the capitu- 

 lum has become reduced, the tubercu- 

 lum is usually only loosely attached, 

 allowing the greatest possible move- 

 ment. In Shastasaurus the single 

 rib articulation of the dorsal region 

 is marked by the development of an 

 angle or prominence on the rib head 

 and on the diapophysis. The artic- 

 ulations are so situated on both faces 

 that .the upper portion of the rib head 

 and diapophysis turn away from each 

 other, leaving a gap which was evi- 

 dently partly filled in with cartilage or connective tissue (fig. 66). The 

 cutting out of space between the upper portion of the rib and the diapophysis 

 permitted a rocking movement of the rib head of such a nature as to allow ex- 

 traordinary elevation of the distal end of the rib shaft, and without reducing the 

 head to abnormally small dimensions. 



A careful examination of the vertebral rib articulation in the cervical and 

 anterior dorsal regions of Shastasaurus and of most specimens of Cymbospon- 

 dylus suggests most strongly that in passing backward through the series the 



66a 



Figs. 66a and 666. Shastasaurus altisjrinus Merrmm. 

 Middle dorsal vertebra and rib, illustrating rib artic- 

 ulation, X %. Fig. 66rt, lateral view of centrum; 

 fig. 666, anteroposterior profile of centrum and rib. 



