378 University of California Publications in Geology [Vol. 7 



PBOSTHENNOPS(?), sp. 



Some months ago the class in field geology, working under 

 the direction of Professor Andrew C. Lawson, obtained in fresh- 

 water shales of the Siestan beds near Bald Peak a specimen 

 representing a portion of the skull of a dicotyline form not 

 previously known from California. The specimen consists of a 

 portion of the cranium showing part of the palate with M 2 and 

 M 3 of the right side, and the left cheek-tooth dentition except- 

 ing M 1 . The teeth are much worn, but enough of the characters 

 are indicated to make possible an approximate determination. 

 The fragment containing the anterior portion of the upper denti- 

 tion of the left side was separate from the portion with the molar 

 teeth when it came into the writer's hands. It seems to fit 



Fig-. 5. Prosthennops( ? ) , sp. Upper cheek-teeth. No. 10826, natural 

 size. Siestan beds, near Bald Peak. 



imperfectly against the anterior end of the piece with the molars. 

 If this diagnosis of the relation of these pieces to each other is 

 correct, a gap approximately fifteen millimeters long between 

 P 4 and M 2 represents the space occupied by M 1 . 



The most anterior element of the cheek-tooth series, con- 

 sidered as P 2 , is double-rooted. The posterior root is wide trans- 

 versely and suggests incipient division into two. The crown is 

 relatively wide posteriorly, and shows a large anterior tubercle 

 with a posterior tubercle or cusp which has an anteroposterior 

 diameter not more than half that of the anterior tubercle. It is 



