250 



University of California . 



[Vol. 3. 



RELATIONSHIPS AND SYSTEMATIC POSITION. 



The Ichthyosauria previously described from the Upper 

 Triassie fauna of California comprise six species placed in the 

 genus Shastasaurus. So far as the extremities are known, these 

 species are characterized by extreme shortness of the limb 

 elements. The humerus, radius and ulna are wider than long, 

 while the phalanges are circular in outline and ordinarily with- 

 out lateral notching. The coracoid is antero-posteriorly con- 

 stricted or pedunculate, the ribs of the middle dorsal region 

 have but one head, and excepting one form (S. perrini) the 

 vertebral centra are distinguished by their shortness. 



Of the two new types, one (Pis. 21, 22 and 23) is character- 

 ized by its elongated limb elements, tridactyle manus, notched 

 or constricted phalanges, shield-shaped coracoid, and long verte- 

 bral centra. For this form the new generic and specific name 

 Leptocheirus zitteli is proposed. 



The second form (PL 24) has also the elongated type of 

 vertebral centrum and the slender tridactyle limbs but differs 

 from the other species, so far as known, in possessing dorsal ribs 

 with widely forked heads. Its limbs differ from those of the 

 other new type in that the posterior extremities are larger than 

 the anterior instead of being much smaller. This form has been 

 given the name Toretocnemus calif ornicus. 



Both Leptocheirus and Toretocnemus are sharply separated 

 from Shastasaurus by their limb structure. The slender pro- 

 podial, epipodial, aud phalangeal elements are wholly different 

 from the greatly abbreviated forms known in Shastasaurus. 

 From each other Leptocheirus and Toretocnemus are distinguished 

 by the character of the rib articulation in the dorsal region and 

 by the difference in the relative size of the limbs. 



Both of the new genera, as also Shastasaurus, differ consider- 

 ably from the European Mixosaurus* In that genus the limbs have 

 elongated propodial and epipodial segments but are distinguished 

 by being pentadactyle, by having four elements in the first row 

 of the mesopodial segment, and by the articulation of two or 



*See E. Repossi. II Mixosauro degli strati triasici di Besano in Lombardia. 

 atti della soc. ital. di scien. natur. Vol. XLI, Passe. 3, p. 361-372, Tav. VII, IX, 

 Nov., 1902. 



