MKRRIA.M: TRIASSK' [CHTHT08AUEIA. it? 



Throughout the larger part of the column, back to the middle of the caudal 

 region, the neural spines are relatively high and slender, but are not greatly 

 broadened anteroposterioiiy. The zygapophyses are distinctly developed (lig. 

 123), but are not particularly large or strong. The writer is uncertain as to 

 whether the zygapophysial facets are separated. 



In the characters of the caudal region Mi.rvwmriiH differs greatly from 

 IchthyoNdtiritN, and to a considerable extent indicates less complete adaptation 

 to aquatic conditions. In its broader outlines the tail seems to differ consider- 

 ably from the caudal region of all later Ichthyosauria. In contrast to the 

 form of caudal fin seen in Irlitliij<tnrnH, in which the distal portion of the 

 vertebral column is rather sharply deflexed, the Mi.rosmtruH specimens avail- 

 able show near the end a gentle upward curvature of the column, which then 

 bends down again and straightens out near the distal portion. ( See fig. 45, p. 

 42, and fig. 50, p. 43.) The structure seen here would evidently offer sup- 

 port to a distal expansion of the tail, and would considerably increase its power 

 beyond that of the form of propelling tail which is commonly found in types 

 which inhabit both shore and water. This fin would possibly have less power 

 than the wider expansion of the double-lobed fin of the typical Ichthyosaurus. 

 Evidently the form seen here marks the transition from the straight, but some- 

 what broadened, tail of a shore dweller to the specialized fluke of the later 

 ichthyosaurs. 



As has been shown by Repossi (1902, tav. 9, fig. 8), the superior arches of 

 the caudal vertebrae near the bend in the tail are extraordinarily high, and 

 tend to take an erect position; those over the posterior portion of the bend 

 inclining forward. This peculiar feature is noticed in those exhibition speci- 

 mens in Milan in which the caudal region is well shown. Its function is 

 evidently the broadening of the caudal fin at the point where the greatest 

 amount of power can be applied to the best advantage. It is paralleled in 

 several other groups of aquatic saurians in which the adaptation has been 

 generally considered somewhat less complete than in the ichthyosaurs. In the 

 mosasaurian genus Clidastes the relations of this portion of the tail region are 

 somewhat similar. (See fig. 51, p. 44). 



In addition to increase of the height of the tail through the elevation of the 

 superior arches we find the centra of a considerable portion of the tail region 

 very much compressed laterally and relatively high (fig. 124). 



As in the American Triassic forms, the lower arches of the caudal region 

 are united ventrally to form long-stemmed chevrons. The form of these 

 chevrons is not unlike that in Delphi no saurus perrini. It is possible that Re- 

 possi 's fig. 9c, tav. 9 (1902) represented as an upper arch is really a chevron, 

 as it duplicates the form of chevrons shown in the specimens in the Milan 

 museum. (See figs. 125 and 126.) It may be noted that the space available 



