76 



University of California Publications. 



[Geology 



OMPHALOSAUEUS NEVADANUS, n. gen. and sp. 

 Pis. 8 and 9, and Text-fig. 1. 



Anterior vertebral centra biconcave. Rami of the mandible 

 rapidly uniting' anteriorly in a strong' symphysis. Splenials 

 broad and heavy. Dentaries united medially only a short dis- 

 tance in front of their posterior extremities, broad superior sur- 

 face set with several rows of very low-crowned crushing teeth. 

 Palatine region with broad edentulous pterygoid elements. Pos- 

 terior portion of the palatines also apparently edentulous. 



The vertebrae present are distinctly amphicoelous and the 

 centra are nearly circular in vertical cross-section. They are in 

 series at the back of the skull, and are evidently anterior cervicals. 



Fig. 1. Omphalosaurus nevadanus, n. gen. and sp.. Outer side of a portion 

 of the right ramus of the mandible. A, angular; Sa, supra-angular; 

 D, dentary ; Sp, splenial. Middle Triassie, marine beds, South Fork of 

 American Canon, West Humboldt Range, Nevada. X %. 



The mandible has lost the articular region and is broken off 

 a short distance in front of the symphysis. The angular, supra- 

 angular, dentary and splenial are represented, with possibly some 

 portions of other elements. The thickness of the jaw back of the 

 symphysis is almost as great as the height. The rami unite in a 

 very wide symphysis only a short distance in front of the poste- 

 rior end of the dentaries. The angular extends unusually far for- 

 ward between the splenial and the dentary. The heavy supra- 

 angular shows no coronoid elevation ; and no discrete coronoid 

 element has been certainly recognized, though possibly present. 

 As will appear from the discussion of the dentition, the develop- 

 ment of a large coronoid elevation would almost have been ex- 

 pected to occur here, as in Placodus. 



