12 MEMOIRS OF THE UNIVERSITY OF CALIFORNIA. 



an interesting evolution from a deltoid Triassic type to the T-shaped element 

 of the Jurassic ichthyosaurs. New material from the vertebral column was 

 also described and figured. 



In 1903 new material from California made possible the description of a 

 complete anterior limb of an American Triassic ichthyosaurian. (Merriam, 

 1903, 1). In this genus (Lcptocliet'ms Merriam Mcrriamia Boulenger) in- 

 stead of the primitive pentadactyl structure of the mixosaurian limb only three 

 complete digits were represented. In another form, Toretocnemus, the hind 

 limb was fully as large as the anterior one, but there were apparently only 

 three digits and a very rudimentary fourth in the podial region. The pelvis 

 of Toretocnemus was plate-like, and the pubis perforated by an obturator fora- 

 men. The middle dorsal vertebrae, unlike those of Shnstasaurus and Delplthi- 

 oKiinrus, supported double-headed ribs. 



The first description of other than fragmentary remains from the Middle 

 Triassic of America appeared in 1895 in the discussion of a finely preserved 

 pectoral arch with the propodial and epipodial limb segments (Merriam, 

 1905, 2). The shoulder girdle of this form differed from that of any other 

 known ichthyosaur, and the limb elements appeared to be the most primitive 

 then known in the Ichthyosauria. 



GEOLOGIC AND GEOGRAPHIC OCCURRENCE. 



The geographic range of the Triassic ichthyosaurs is remarkably wide al- 

 most as far back as we can trace the group. Even from the Middle Triassic, 

 remains are known from both Europe and western North America. From the 

 Upper Triassic they are known from these regions, and possibly from others 

 also. 



Europe. The oldest known remains of Ichthyosauria are those from the 

 lower portion of the Muschelkalk of Germany and Switzerland. Fraas (1891, 

 p. 37) states that the localities in the Swabian Muschelkalk at which specimens 

 have been found represent a very constant horizon in the Wellendolomit. Un- 

 fortunately the material obtained consists only of isolated vertebrae, jaw frag- 

 ments, or other separated parts of the skeleton. As yet no complete skeletons 

 have been discovered. 



A small amount of material from the lower Muschelkalk of southwest Ger- 

 many and northern Switzerland is preserved in the collections of the Koenig- 

 lichen Naturalien-Cabinet, and in that of the Technische Hochschule in 

 Stuttgart, in the Geologisch-Mineralogisches Institut in Tuebiiigen, and in the 

 Polytechnikum in Zurich. A few specimens of ichthyosaurian vertebrae from 

 the lower Muschelkalk in North Germany are preserved in the geological Mu- 

 seum of Halle a. S. Of the Swabian localities where material has been found in 



