34 



University of California Publications. [Geology 



nearly as can be determined, these beds seem to be somewhat 

 older than those from which the Mixosaurs of the Italian Trias 

 have been obtained. From the limestone horizon exposed in 

 Cottonwood Cailon two saurian species have been described by 

 Leidy.* Both were founded upon portions of vertebrae neither 

 the generic nor specific characters of which have yet been satis- 

 factorily determined. The most common ichthyosaurian species 

 in these beds shows a strong similarity to the imperfect speci- 

 mens designated as Cymbospondijlus petrinus by Leidy. The 

 specimen described here is tentatively referred to that species. 



The greater number of the Ichthyosaurians showing primi- 

 tive limb structure which have been described up to this time 

 have been very small forms, and a suggestion of doubt may rea- 

 sonably have been entertained concerning the probability of large 

 forms from the same beds showing these archaic characters to 

 the same degree. The earlier representatives of most groups are 

 small, and in such forms primitive characters appear frequently 

 when they would not be present in larger members of the same 

 group. It is, therefore, a matter of some significance that the 

 specimen under consideration ranks among the larger Ichthyo- 

 saurs, the sludl measuring at least a metre in length and the 

 humerus 280 mm. in length. 



The elements of the pectoral arch and the limb were found 

 in nearly the position in which they are figured, PL 5, excepting 

 the scapula, which had been partly inverted. 



The 'pectoral arch differs from that of all ichthyosaurian 

 genera heretofore described. The coracoid is sickle-shaped, with 

 a convex anterior border and an acute posterior angle. A fora- 

 men of considerable size is situated a little in front of the prox- 

 imal articulation. This element differs from the coracoid in 

 Mixosaurus and in Shastasaurus in possessing a convex anterior 

 border, while in both of the other genera the proximal end is 

 narrowed or pedunculate and the anterior border concave in part. 

 Excepting the presence of a distinct perforation, the form is 

 much the same as in the coracoid of the recently described Delphi- 

 nosaurus-f from the Upper Triassic of California. 



*See Bull. Dept. Geol., Univ. Calif., Vol. 3, No. 4, p. 104. 

 tAm. Jour. Sei., Vol. 19, p. 24, Jan., 1905. 



