MERRIAM: TRIASSTC IOTTTITYOSAURTA. 41 



land (fig. 48) the lower arc-lies of the caudal series appear to be larger and 

 extend out in the series to a point considerably beyond the bend in the tail. 

 In these forms the area of the paddles is generally larger, and in many cases 

 the posterior paddles are relatively large. 



Among the Triassic ichthyosaurians the terminal portion of the caudal 

 series is known in Mixomunm, Di>1i>ltiiu>K units, and to a considerable extent 

 in Cymbospondylus. Caudal vertebrae are also known from the other genera. 



In Mixosaurus the well-preserved specimens at Milan show a structure which 

 resembles that of Ichthyoxd/irtts sufficiently to call to mind the type of tail fin 

 present in the latter genus, but different enough to merit discussion as a dis- 

 tinct type (fig. 45). In this form the distal end of the caudal region was 

 apparently not bent downward sharply as in the latest ichthyosaurs, but curved 

 slightly upward near the middle of the caudal series and then down, again some 

 distance in front of the posterior end of the tail (fig. 50). This difference 

 would not of itself attract attention so strongly were it not that the character 

 of the vertebral arches in the flexed portion of the tail is very different from 

 that in Ichthyosaurus. The neural arches instead of suffering gradual reduc- 

 tion as the point of curvature is approached, and disappearing very close 

 behind it as in Ichthyosaurus, are considerably increased in length and stand 

 erect in the curved region, and continue far back toward the terminal region 

 of the series. In the Milan specimen best known to the writer the caudal region 

 consists of about 76 vertebrae. The upper arches begin to turn forward at 

 about the seventeenth centrum behind the posterior limb. The hypocentral ele- 

 ments are long, the inferior ends of the right and left pieces uniting to form 

 Y-shaped arches. The writer has not seen lower arches immediately below the 

 bend in the caiidal series, but they are present and well developed behind 

 the bend. 



In a small specimen occurring in the same beds with Cymbospondylus in 

 the Middle Trias of Nevada (no. 10624, fig. 44) practically the same relations 

 of the elements of the caudal vertebrae appear as are found in Mixosaurus. 

 The caudal centra are somewhat flattened laterally and bear apophyses, with 

 which caudal ribs may have been in articulation. The neural arches are high 

 and slender, their height equaling a little less than three times the height of 

 the centra (X2%). They show little if any anteroposterior curvature and 

 stand nearly erect on the centra. The inferior arches are also long and 

 slender, but are strongly recurved. Their length equals nearly two and one- 

 half times the height of the centra. The articulation of each of the infe- 

 rior arches seems to be mainly on a single centrum, instead of intervertebral. 

 The union of the lower arches with the centra is so close that it may possibly 

 amount to co-ossification in some cases. 



In a large specimen of Cymbospondylus petrinus (no. 9947, fig. 46) there 



