F: TRIASSIC ICHTHYOSAUBIA. 1o:! 



from the jaws of Ichthyosaurus. In the eoronoid region no distinct process 

 is developed, but there is some indication of a flattened area on the superior 

 margin as in Cymbospondylus. 



Dent ih'oii. In -the characters of its dentition Mixosaurns seems to repre- 

 sent a peculiar ichthyosaurian type. As was shown in part by Baur, the den- 

 tition is characterized by the relatively small number of differentiated teeth 

 set in the narrow alveolar margins. In an upper jaw figured by Repossi (see 

 tigs. 13 and 14, p. 28) there must have been about twenty to twenty-three teeth 

 present. On a lower jaw there have been forty or more. In Irltflt/joNtnirttx 

 Fraas places the number for long-jawed forms at about fifty teeth per jaw. 

 In all of the specimens shown, the teeth are set in open order, particularly in 

 the anterior portion of the jaws, where they may be separated by a space 

 equaling from two to five or six times the diameter, of the basal portion of their 

 crowns. Many of the roots are in distinctly separated alveoli. The anterior 

 teeth of both jaws are long and slender. Toward the back of the jaws the 

 crowns become lower and their basal diameter increases somewhat. The most 

 posterior crowns may have a basal diameter twice as large as that of the an- 

 terior teeth, and a crown only half as high. Repossi 's figures do not indicate 

 the presence of a large amount of cement on the roots, nor is there indication 

 of folding of the basal portion of the dentine as in M. ( ?) atavus. The enamel 

 of the crowns seems in some cases to be sharply striated. 



Classification. As yet only one species, M. cornalianus, has been described 

 which can be referred with certainty to this genus. Most of the other forms 

 which have been referred to Mixosaurus might with equal right be placed in 

 some other genus. It is not improbable that some of the Italian specimens 

 now available will be found to vary specifically from the type of Bassani's 

 cornalianus. 



CYMBOSPONDYLUS Leidy. 



Cynibospondylus Lcidy, Proc. Philad. Acad. Nat. Sc., vol. 20, p. 177, 1868. 

 Cynibospondylus (Merriam), Univ. Calif. Publ. Geol., vol. 3, p. 104, 1902. 

 Cynibospondylus (Merriam), Univ. Calif. Publ., vol. 4, p. 33, 1905. 



Presacral region of vertebral column including about 65 vertebrae. Distal 

 end of caudal region slightly decurved. Neural arches turned forward in re- 

 gion of curvature. Vertebral centra of distal caudal region much compressed 

 laterally. Caudal vertebrae with Y-shaped chevrons. Rib articulation single- 

 headed excepting in the anterior region of the vertebral column. 



Anterior arches and limbs but little larger than posterior. Interclavicle 

 unknown. Humerus, radius, and ulna relatively long and narrow, ulna and 

 radius both with median constriction. Digits unknown. Inferior pelvic ele- 

 ments broad, plate-like. Femur somewhat more primitive than humerus. 



