MKRKIA.M: TK'IASSU' ICJITII YOSAl'h'I A. 4.1 



Mixosaurua appear in other groups, particularly in the inosasauriaii genus 

 Clidastes (fig. 51), in which a portion of the caudal region somewhat in advance 

 of the posterior end of the series is considerably broadened by the elongation 

 and erection of the neural arches. The inferior spines are also somewhat 

 elongated, and although fused to individual vertebral centra they are quite 

 strongly recurved. 



The modification of the caudal fin of ClidttslcN differs from that of Mi.ro- 

 Mttirux mainly in showing less evidence of vertical curvature of the distal 

 portion of the vertebral column. If the tail fin of Cymbospondylu^^T) iHtfaii* 

 were better known it would possibly be found to differ from Clidastes in general 

 character less than does the corresponding region in the caudal series of Mi.ro- 

 saurus. 



In the caudal fin of the typical Cymbospondylus the centra become higher 

 and narrower beyond the bend, instead of smaller as in later forms, while the 

 upper and lower arches are retained beyond the bend. The upper arches are 

 inclined forward as in Mixosaurus, but are not extraordinarily elongated as 

 in that genus and in C. ( ?) natans. Vertical expansion of the fin is produced 

 in this instance partly by retention of the upper and lower arches, partly by 

 increased height of the vertebral centra, and partly by vertical curvature of 

 the column. 



In the known history of the Ichthyosauria the caudal fin has passed through 

 several fairly definite stages of evolution, and the trend of modification has 

 always been in practically the same direction. The most primitive structure 

 is probably that of C. ( ?) natans which leads to the type of Mixosaurus. In 

 the large Cymbospondylus the structure is almost as primitive as in the other 

 two Middle Triassic forms, the differences being possibly due in part to dif- 

 ference in size of the individuals compared. 



Arranged in accordance with the hypothetical scheme of evolution of the 

 ichthyosaurian caudal fin, the various know r n stages in the evolutionary series 

 are, as shown on the following page, also arranged in the order of their appear- 

 ance in time ; excepting in the case of those Middle Triassic forms in which the 

 factor of size is also to be considered. In the case of these Middle Triassic 

 forms size has exerted the influence which could generally be predicted, the 

 smaller forms showing the most primitive structure. 



The fact that the known forms fall into a fairly definite series is not neces- 

 sarily to be taken as indicating a single direct line of descent. They are 

 widely scattered geographically, and differ sufficiently in minor details of 

 structure to make difficult their arrangement in a single series. The fact that 

 wherever the forms of a given period are found they seem to fall into this pre- 

 dicted position in the evolutionary scheme indicates all the more clearly the 

 definite and long-continued tendency of evolution of the group in this particular. 



