Meruiam.] 



Ne w Ic hih y out uria. 



251 



move distal carpals on the intermedium. The arches, vertebrae, 

 and dentition are also, as far as known, different from the 

 corresponding- structures in the Californian forms. From 

 Ichthyosaurus the new genera differ in the form of their pectoral 

 and pelvic arches, the elongation of their epipodial segments, and 

 the constant notching with frequent double notching or shafting 

 of their phalanges. 



Including the forms here described, six distinctly separated 

 groups of Ichthyosauriaus designated as genera are now known. 

 These groups may be characterized as follows: 



Mixosaurus. Dorsal ribs mainly single-headed. Coracoid 

 pedunculate or with anterior and posterior emargination. 

 Scapula, ischium, and pubis much expanded distally. Inter- 

 clavicle triangular, with concave borders. Limbs pentadactyle, 

 propodial and epipodial segments elongated. Elements of epipo- 

 dial segments separated by a wide cleft. Phalanges notched, 

 sometimes with double notching or shafting. Four elements in 

 the proximal row of mesopodial region. Intermedium supporting 

 at least two elements distally. Dentition differentiated. 



Leptocheirus. Dorsal ribs, so far as known, single-headed. 

 Coracoid elliptical, without emargination; scapula expanded 

 distally; clavicle broad; interclavicle probably broadly triangular. 

 Limbs tridactyle with a very rudimentary fourth digit. Pro- 

 podial and epipodial segments elongated. Elements of epipodial 

 segments separated by a wide cleft. Phalanges notched on one 

 or both sides. Three bones in the proximal row of mesopodial 

 segments. Carpus and tarsus with linear arrangement, the 

 intermedium supporting a single element distally. 



Shastasaurus . Ribs of dorsal region mainly single-headed. 

 Caudal hypocentra uniting to form long chevron bones. 

 Coracoid pedunculate; scapula, ischium, and pubis much expanded 

 distally. Pubis with deep obturator notch. Elements of pro- 

 and epipodial segments greatly shortened. Radius and ulna 

 separated by a cleft. Ulna considerably smaller than radius. 



Cymbospondijlus (?) As yet, too imperfectly known for 

 satisfactory generic description. 



Toretocnemns . Ribs of the middle or posterior dorsal region 

 with widely forking heads. Caudal hypocentra uniting to form 



