MERRIAM: TKTARSIC ICHTHYOSAURI A. xil 



ittnintismay be associated with either of the two families; it may he subdivided 

 into two quite distinct groups and associated with both; or it may be consid- 

 ered ancestral to both. 



The phylogenetic scheme on the preceding page illustrates the broader rela- 

 tionships of the ichthyosaur groups, as they may be assumed to be situated on the 

 basis of a primitive bicipital rib articulation. 



The major divisions of the Ichthyosauria may be denned as follows: 



ICHTHYOSAURIA. 

 MIXOSAURIDAE. 



Articulation of dorsal ribs generally unicipital. Hypocentral elements of 

 the caudal region uniting to form long-spined chevrons. Distal region of 

 caudal series not strongly decurved. Zygapophyses in anterior half of ver- 

 tebral column consisting of distinctly separated lateral faces. Number of 

 presacral vertebrae may reach 65. Pelvic elements plate-like, relatively large. 

 Scapula expanded distally. Epipodial elements relatively elongated, and 

 separated by a cleft (excepting in Shastdsaurus careyi). Hyperdaetyly not 

 developed. Hyperphalangy less marked than in Ichthyosauridae. Phalangeal 

 elements frequently elongated and shafted. Facial region shorter and max- 

 illary longer than in Ichthyosauridae. Orbits relatively small in most forms. 

 Postorbital bar not markedly narrowed. Teeth few to numerous, isodont or 

 differentiated, in open grooves or in pits. 



Mijcosaurinae.' Presacral vertebrae 48-50 (?). Cervicals with unicipital 

 rib articulation (?). Distal region of caudal series curved upward anterior 

 to the posterior end, and slightly decurved distally; neural spines elongated 

 and erected in the region of curvalure. Pubis without obturator notch or 

 foramen. Interclavicle with expanded median stem. Podial portion of pad- 

 dles relatively long and broad, five digits present. Orbits moderately large. 

 Teeth well spaced, set mainly in pits, differentiated. 



Shastasaurinae. Presacral vertebrae 45 ( ?) to 65. Cervicals with bicipital 

 rib articulation. Distal region of caudal series as in the Mixosaurinae, or 

 somewhat decurved as in the Ichthyosauridae. Pubis with obturator notch or 

 foramen. Paddles with relatively short and narrow podial region. Digits 

 where known reduced to three or four. Orbits small to large. Postorbital 

 bar not greatly narrowed. Teeth numerous, close set in open grooves or in pits. 



ICHTHYOSAURIDAE. 



Articulation of the presacral ribs mainly bicipital. Hypocentral elements 

 of the caudal region sometimes absent, when present not meeting to form long- 

 spined chevrons. Distal portion of caudal series sharply decurved and enclosed 



