.MKRHIA.M: TRIASSIC ICHTHYOSAUHIA. (i:5 



der. The clavicles are large in both forms. The interclavicle of Cliii>ii]>uosaur- 

 IIN is T-shaped as in Ichthyosaurus, hut this element is unknown in ('i/nib(^i>oii- 

 dylus. 



Tlie type of arch seen in the Triassic ichthyosaurs seems to approach most 

 closely the form seen in the Diaptosauria, but owing to important changes in the 

 functions of the anterior limb in adaptation to natation, it has come to vary con- 

 siderably from that type in most cases. 



LIMBS. 



As early as the Triassic the amount of differentiation shown in the limb 

 structure of the Ichthyosauria is surprisingly large, and apparently indicates 

 that several fairly distinct lines of evolution or adaptation had been laid down. 

 Excepting the greatly specialized shastasaurian species of the uppermost 

 portion of the Upper Trias there is, however, in all of these varied forms an 

 interesting group of characters which seem to interpret themselves only as per- 

 sisting features of a primitive littoral ichthyosaurian with extremities having 

 functions quite different from those of the true Ichthyosaurus. 



In general the limbs of Triassic, ichthyosaurians are distinguished by the 

 following characters (see figs. 90-101) : the epipodial elements are relatively 

 long and are separated by a median space; in one or both of the epipodials 

 there is a tendency to show a median constriction ; the phalangeal region of the 

 limb is relatively short; hyperdactyly never occurs; hyperphalangy is much 

 less pronounced than in the later forms; the individual phalanges are rela- 

 tively long, they are commonly notched on one or both borders even in the 

 digits not situated on the borders of the limb, and may in many cases show a 

 median constriction or shaft portion. Though it is not true of several of the 

 genera, the hind limbs are generally relatively large. In Toretocnemus they 

 appear to be fully as large as the anterior limbs. In Cymbospondylus and 

 Shastasfturux they seem to have been almost as large. 



While most of the above characters appear together in nearly all of the 

 known Triassic forms, there have been developed in the Triassic some limb 

 specializations which have eliminated certain of the primitive characters just 

 mentioned. This is particularly true of Shastasaurus, which is, however, a 

 very late Triassic form, and also represents a peculiar line of limb speciali- 

 zation apparently not present in the Jurassic genera. 



As nearly as can be determined, at least two types of limb structure are 

 represented in the Triassic genera. One form is seen in the broad paddle of 

 Mijrosa-urufi with a long podiaP region, four proximal mesopodials, an alter- 

 nating arrangement of the first and second rows of mesopodials, and five digits. 



" The word ' ' podial ' ' is here intended to represent the metapodial and phalangeal regions. 



