132 MEMOIRS OF THE UNIVERSITY OP CALIFORNIA. 



bral centra reach to the posterior part of the skull. Beneath the skull, weath- 

 ering and imperfect preservation of the bones make it difficult to determine 

 how many centra have been present, but there could hardly have been more 

 than ten, and the number was probably somewhat less. The total number of 

 vertebrae for the presacral region including an estimate of between five and 

 ten for centra possibly lost beneath the skull would be near 45, or about the 

 same as in Ichthyosaurus. In the type specimen there are 35 presacrals 

 present, and the estimated number for the whole presacral region is also 

 about 45. 



The caudal region of no. 10998 terminates with a series of very small centra 

 about ten millimeters in height. It appears to be nearly complete, though a 

 few more terminal centra may have been present originally. In the distal half 

 of the caudal region, following a slight downward curvature, the tail is bent 

 upward and down again as in Mixosaums, and at the extreme distal portion 

 it is curved upward again. As the whole vertebral column is turned partly 

 over, and is somewhat disturbed in this specimen, it may perhaps be ques- 

 tioned whether the curvature seen here represents the normal position of the 

 vertebral column in the caudal fin. There is, however, relatively less disar- 

 rangement of the vertebral centra in the caudal region than elsewhere, and 

 a part of the sinuosity shown may represent a normal vertical curvature, some- 

 what resembling that of Mixosaurus. 



In the vertebral column of the -type specimen of D. pen-hit there are be- 

 tween 25 and 30 vertebrae in that portion of the caudal region anterior to the 

 bend of the tail. In specimen 10998 the beginning curvature seems to occur 

 between the 25th and 35th caudals. 



Throughout the whole vertebral column the centra are extraordinarily long 

 for ichthyosaurians. In a middle dorsal of the type specimen the ratio of 

 height to length is 1 :84, and in an interior caudal 1 :56. 



The spines of the upper arches are not greatly elongated in any part of 

 the column, but are in general considerably compressed laterally (see fig. 32 

 and 32&, p. 34), particularly in the dorsal and anterior caudal regions. The 

 strong lateral compression of these arches stands in decided contrast to the wide 

 cross-section of the neural spines of the dorsal region in some forms of Shas- 

 tasaurus. That the flattened form of spine is derived from an originally 

 thicker type, is suggested by the distinct lateral rib situated about the middle 

 of the expanded blade of each arch. 



The rib articulation is evidently similar to that of Shastasaurus. Though 

 little is known of the most anterior cervical vertebrae, they seem to have sup- 

 ported bicipital ribs. From the anterior dorsals back to the caudals the ribs 

 were single-headed. In the posterior dorsal and anterior caudal regions the 

 rib heads are well preserved, and there is no evidence of bifurcation of the 

 heads, as has been suggested for Mixosaurus. 



