252 



University of California. 



LVol. 3. 



cheveron bones. Ischium and pubis much expanded distally. 

 Pubis with obturator foramen. Limbs tridactyle with rudi- 

 mentary fourth digit, structure in general as in Leptocheirus. 

 Posterior limbs larger than anterior. 



Ichthyosaurus. Ribs of dorsal region double-headed except- 

 ing a few of the most posterior. Coracoid generally pedunculate 

 or with emargination. Scapula, ischium, and pubis narrow. 

 Bones of the epipodial segments greatly shortened, generally in 

 contact along their inner borders. Notching present on only a 

 few phalanges on the anterior border of limb. Number of digits 

 varying from three with a rudimentary fourth to ten. Dentition 

 isodont. 



Baptanodon, Opthalmosaurus . Propodial elements articulat- 

 ing distally with three greatly shortened epipodial bones. 

 Dentition considerably reduced. 



The subdivision of the order Ichthyosauria into families was 

 attempted by Baur,* who set up a family for each of the genera 

 known at that time, viz., Mixosaurida?, Ichthyosaurida?, and 

 Baptanodontidae. To these three the writer has suggested the 

 addition of the Shastasaurida?t as a group sharply distinguished 

 from the others. Repossi's recent paper shows Mixosaurus to be 

 much more like Shastasaurus than was indicated by previous 

 descriptions. The two genera are, however, distinctly separated, 

 the types of limbs particularly being widely different. Of the 

 two new genera here described Leptocheirus is nearer to Mixo- 

 saurus than to any other of the genera but has throughout a 

 different type of extremity and pectoral arch, while the dentition 

 shows no signs of differentiation. Toretocnemus is like Lepto- 

 cheirus in its type of limb, like Shastasaurus in the form of its 

 pelvic arch and chevrons, and like Ichthyosaurus in the forking 

 of its dorsal ribs. 



The question as to how the genera in the order Ichthyosauria 

 should be grouped was one which did not seem difficult to solve 

 when only Ichthyosaurus, Baptanodon, and Shastasaurus were 

 fairly well known and but a small part of the skeleton of Mixo- 

 saurus had been described. Now that Mixosaurus has been 



*G. Baur, Ueber den Ursprung der Extreniitaten der Iehthyopterygfa, Ber. d. 

 XX Versam. d. Oberhein. Geol. Ver. Vol. XX, p. 3. 

 *Bull. Dept. Geol. Univ. Calif., Vol. 3, p. 87. 



