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middle caudal region (fig. 128). Excepting the dis- 

 location of some of the anterior vertebrae, and a 

 small gap above the pelvis, the series is unbroken. 

 Out of the total number of 84 vertebrae present, 67 

 are evidently presacrals. The location of the sacral 

 region is pretty definitely fixed, both by the position 

 of the pelvis and by the reduction in length of the 

 ribs. The elements of the pelvis are still in their 

 natural positions with reference to each other, and 

 the ilium on one side appears to have almost its nor- 

 mal relation to the vertebral column. Considering 

 that these elements, as also the vertebral centra and 

 arches, and the ribs, show little evidence of disturb- 

 ance, it is fair to assume that the vertebra above the 

 ilium was either next the point of attachment of the 

 pubis or close to it. The ribs in this region are still 

 of considerable length, and if the point of attach- 

 ment is not located at this place it is probably some- 

 what farther back. 



The number of presacral vertebrae present here 

 exceeds considerably that of any other ichthyosau- 

 rian of which the writer has any information. In 

 the typical Ichthyosaurus the number is ordinarily 

 near 45. The extraordinarily large number of 54 

 presacrals is indicated recently by Broili"' 4 in /. ]>/<>- 

 tydaeti/lus. In Mixosaurus the number seems to have 

 been between 45 and 50. The number in C. {H'trimix 

 is also larger than that in any other marine saTirians 

 excepting very long-necked forms, and suggests the 

 possibility that the number of presacrals may have 

 been reduced rather than increased in the later Ich- 

 thyosauria. A general reduction of the number in 

 later forms might be correlated with the development 

 of a highly specialized caudal fin, causing localiza- 

 tion of the locomotor function in the distal caudal 

 region, instead of distributing it through nearly the 

 whole length of the tail. 



In specimen 9950 there are eleven vertebrae in 

 front of the anterior border of the coracoid and 

 scapula. Although no definite determination of the 



si Broili, P., Palaeontog., Bd. 54, Taf . 13. 



