00 MEMOIRS OF THE UNIVERSITY OF CALIFORNIA. 



in the lower portion of a two-lobed sculling fin. Zygapophyses consisting of 

 a single anterior or posterior face. Presacral vertebrae about 45 in number. 

 Pelvic elements much reduced. Scapula narrow distally. Hind limbs gener- 

 ally much reduced. Epipodial elements very short and not separated by a 

 cleft. 4 " Hyperdactyly frequent. Hyperphalangy very pronounced. Phalan- 

 geal elements polygonal or rounded. Facial region relatively long. Maxillary 

 relatively short. Orbits very large. Postorbital bar generally narrow. Teeth 

 numerous, isodont, in open grooves. 



Ichthyosaurinae. Hypocentral caudal elements usually present. 44 Inferior 

 pelvic elements separate or fused. Epipodial region of anterior limbs including 

 only two elements. 4 ' 1 Epipodial bones angular in outline. Phalangeal elements 

 polygonal, sometimes notched on the anterior side in the anterior row. Teeth 

 numerous. 



Baptanodontinae. Hypocentral elements reduced, or absent from caudal 

 region. Ilium and ischium fused. Epipodial elements three in number in the 

 anterior limb. All limb elements excepting the propodial and anterior epipodial 

 are discoidal in form, and with excavated or pitted margins. Orbits very large, 

 and postorbital bar very narrow. Teeth few and small. 



SYSTEMATIC DESCRIPTIONS. 



MIXOSAUBUS( ?) ATAVUS (Quenstedt). 



Ichtliyosaimts atavtis Quenstedt. Petrefactenknnde, Aufl. I. 1852. p. 129; Aufl. II. IStiT. 



p. 160, Taf. 6, fig. 7-10. 

 Mixosaurus atavus, major and minor (Fraas). Ichthyosaurier d. Siidentschen Trias- 



and Jura- Ablagerungen, 1891, p. 37, Taf. 1, fig. 17 and 18 ; Taf. 3, fig. 1-16. 

 IclithyoftaurusC!) atavus (Merriam). Am. Jour. Sc., vol. 19, p. 30. Jan., 1905. 



To this species Quenstedt referred a number of fragmentary Triassic speci- 

 mens evidently representing ichthyosaurians. In the absence of distinguishing 

 characters it was included in the genus Ichthyosaurus. In additional material 

 of similar type Fraas recognized characters separating this form from Ich- 

 thyosaurus, and included it in the genus Mixosaurus, which had been described 

 by G. Baur from a study of the Italian Triassic material. More recently 

 Merriam has questioned the generic identity of this form with the species re- 

 ferred to Mixosaurus. 



Vertebral Column. The vertebrae of this form are known through a 

 comparatively few specimens, nearly all of which are represented by centra 

 without arches. 



43 Very exceptionally slightly separated. 



44 See page 46. 



4 Rarely, as in Ichthyosaurus extremus, with more than one element. 



