24 J. C. Merriam — Triassic Ichthyosauria. 



alexandrae. The fourth appears in a recently discovered 

 specimen which seems to be specifically identical with Shasta- 

 saurus perrin i. This form evidently represents a genus distinct 

 from the type seen in S. osmonti and alexandrae, and the name 

 Delphinosaurus* may be used to distinguish it from the more 

 specialized species. 



In Mixosaurus (tig. 1) the extremities are of a primitive 

 type. Both man us and pes are pentadactyle,-f- the elements of 

 the limbs are generally quite slender and in many cases have a 

 median constriction. This form is farther characterized by 

 the articulation of the intermedium distally with two or more 

 elements and by the frequent presence of a fourth element 

 (pisiform) on the posterior end of the proximal row in the 

 mesopodial region. 



Toretocnemus and Merriamia do not differ greatly in limb 

 structure though the vertebrae are of distinct types. In Mer- 

 riamia (tig. 2) the limb has but three digits with the merest 

 vestige of a fourth. There are but three elements in the first 

 row of the mesopodial region and both carpus and tarsus are 

 of a strictly linear type, the intermedium articulating w T ith but 

 a single element distally. In this genus the posterior limbs 

 are much smaller than the anterior. In Toretocnemus the 

 posterior limbs equal or exceed the anterior in size and the 

 vestigial fourth digit of the posterior limb appears to have 

 been larger than in Merriamia. 



The most specialized limb found in the Triassic genera, and 

 one of the most specialized types known in the Ichthyosauria, 

 is seen in Shastasaurus osmonti (fig. 4). The anterior limb 

 in this genus is characterized by extreme shortening of all the 

 elements. The humerus is actually as broad as long and is 

 one of the shortest propodial elements known in the reptilia. 

 The epipodial bones are also greatly abbreviated, though sepa- 

 rated by a narrow cleft. Of the carpus only the radiale is 

 known. It is as large or larger than the ulna and its margin 

 is entire, while that of the radius shows an anterior notch. 



In the recently discovered anterior limb referred to Delphi - 

 nosaurus (Shastasaurus) jperrini (fig. 3) the humerus, radius 

 and ulna are longer than in S. osmonti, and the radius and 

 ulna are both deeply constricted. The radiale is narrower and 

 is notched. The carpus is of the linear type and the posterior 

 of the three linear series consists of somewhat smaller bones 

 than are seen in the other two. The elements of the meso- 



* Delphinosaurus is characterized by much elongated vertebral centra ; an 

 unnotched scapula, and the peculiar structure of the limbs described 

 above. 



f See redescription of Mixosaums, E. Eepossi, Atti. della soc. ital. d. 

 scien. Natur., vol. xli, fasc. 3, p. 361-872, Tav. viii, ix. 



