102 MEMOIRS OF TITE UNIVERSITY OF CALIFORNIA. 



The clavicle is slender compared with the corresponding element of Cym- 

 bospondylus. The median end in contact with the interdavicle is considerably 

 expanded, but the middle and lateral portions arc very slender. The median 

 stem of the interdavicle is much more expanded laterally than in the other 

 known genera, and gives to the element as a whole a nearly triangular form. In 

 other ichthyosaurians the median arm is narrow and the element takes on the 

 T form. From beds referred to the lower Muschelkalk, Von Huene (1902, p. 

 11) has described a more robust and more nearly perfectly triangular inter- 

 clavicle. (See fig. 81, p. 60.) The form of this element suggests its possible 

 origin for the median plate in the stegocephalian pectoral arch. 



Without more definite evidence as to their position it is not possible to make 

 certain of the identity of the two inferior elements of the ]>c/rir t/irdh'. (See 

 fig. 71, p. 56.) Neither element shows the obturator notch or foramen present 

 in all of the other Triassic genera. As shown in the figure the bone labeled 

 Pn? resembles the ischium of Delphinosaurus, but is also like the pubis of 

 Toretocnemus minus the pubic foramen. The element /.sf approaches the 

 form of the ischium in Toretocnemus and Shastasaums, and is also like the 

 pubis of Delphinosaurus or Shastasauriis minus the anterior hook. Both in- 

 ferior elements are much expanded, as in the other characteristic Triassic 

 ichthyosaurian genera. The ilium is relatively short and is somewhat widened 

 distally as in most of the Triassic forms. On the whole the pelvis shows as 

 close a resemblance to that of Cymbospoiidi/his as to that of any of the Triassic 

 genera. 



Skull. The principal features of the cranium of Mixosaurus have been 

 represented by Repossi, but as yet the individual elements have not all been de- 

 scribed in detail. The skull is as a whole relatively large compared with the 

 trunk. If any estimate can be made from the nearly complete specimen fig- 

 ured by Repossi (1902, tav. 8, fig. 1), the length of the skull equaled about 65 

 per cent, of the length of the trunk. In Cymbospondylus the length equals 

 about 36 per cent. ; in Ichthyosaurus 4 " it may equal 80 per cent. The facial re- 

 gion is elongated and evidently slender, but is not developed proportionately 

 as in the later ichthyosaurs. The superior narial openings are situated much 

 as in Ichthyosaurus. The orbits are large, though possibly somewhat smaller 

 than in Ichthyosaurus. The rostral region is formed largely of the premaxil- 

 laries, though the maxillaries are relatively larger than in most ichthyosaurs. 

 Repossi 's figures seem to indicate that the nasals are greatly expanded as in the 

 other genera. A considerable number of sclerotic plates in the orbit show the 

 existence of a large orbital ring. The elements of the posterior portion of the 

 skull are not clearly shown. 



The long and slender mandibular rami do not appear to differ materially 



7. integer. See Fraas, E., 1891, Taf . 7. 



