40 



MEMOIRS OF THE UNIVERSITY OF CALIFORNIA. 



of the more generalized shore or fresh-water forms has been evident, but com- 

 paratively little information regarding the course of its evolution has been 

 available. 



In a specimen of Ichthyosaurus trifjoints jtoatliHniufs AVagner from the 

 Upper Jurassic, one of the most recent of the well-preserved ichthyosaur 

 specimens 21 exhibiting the impression of the epidermal investment of the caudal 

 region, the caudal fin is seen to be broadly two-lobed with the distal end of the 



vertebral column turned sharply 

 downward into the lower lobe (fig. 

 42). Traces of upper arches are seen 

 on the vertebrae in the lower lobe of 

 the fin, but lower arches seem to have 

 been absent. The vertebral centra of 

 the tail fin are very much smaller 

 than those immediately in advance of 

 the bend. They are also relatively 

 very much longer antcroposteriorly, 

 while the median portion is consider- 

 ably constricted. The margins of the 

 centra are thickened and rounded. 

 Practically the same relations of the 

 caudal vertebrae are seen in /. ]>l(ity- 

 ddctylHti BroilP of the Cretaceous. 

 In some of the best specimens of Ich- 

 thyosaurus from the Upper Lias of 

 South Germany (fig. 49), the down- 

 ward bend is very pronounced, while 

 the upper and lower vertebral arches 

 are mch reduced. The centra be- 

 yond the point of strongest curvature 

 are not relatively much if any elon- 

 gated. In these types the portion of the tail included in the terminal fin is 

 large and the fin itself nrnst have been capable of exerting considerable power. 

 It was not supported to any extent by spinous projections of the upper or 

 lower arches, but obtained its width largely through the bend in the caudal 

 series. The paddles in these forms were too small to have furnished much 

 power in swimming, and the hind paddles were so reduced as to be practically 

 functionless. 



In a number of the species of Ichthyosaurus from the Lower Lias of Eng- 



^' Bauer, F., Palaeontog., Bd. 44, 1897-98, p. 309, and Taf. 27. 

 22 Broili, F., Palaeontog., Bd. 54, p. 147. 



Fig. 42. Ichthyosaurus trigonus posthunms Wagner. 

 Posterior caudal region with outline of the caudal 

 fin, about 1^0 natural size. (After Bauer.) 



