Geol.— Vol. II.] ANDERSON— CRETACEOUS DEPOSITS. 79 



peripheral angle; tubercles elongated and narrow, standing in single rows on 

 either side of the ventral surface, and opposite one another. The ribs are 

 low and rounded, and about equal in width to the intervening furrows. On 

 old shells they reach the number of about forty on an entire whorl, while on 

 younger shells the number is generally less. The ribs incline strongly for- 

 ward on leaving the umbilicus, but about the middle of the shell describe a 

 sharp curve backward, followed by a more gentle forward curve on approach- 

 ing the marginal tubercles. Upon the periphery the space between the rows 

 of tubercles is flattened and band-like, being equal in width to one-third the 

 thickness of the shell. The early stages of this shell have been described by 

 Dr. J. P. Smith, ^ and its relations to the next species stated. 



Hitherto the genus Placcnticeras has been but little known 

 in the Cretaceous of the Pacific border. Two allied species 

 have recently been recognized in the Lower Chico beds in 

 widely separated districts in California and Oregon. In 

 the above named species the shell is of moderate size, the 

 largest specimen having the following dimensions: — 



Diameter 120 mm. 



Height of last coil 58 mm. 



Width of last coil 30 . 5 mm. 



Width of umbilicus 23 mm. 



Involution 13 mm. 



Occurrence. — This shell is known from the Lower Chico 

 of Phoenix, Henley, Arroyo del Valle, and the San Fernando 

 Mountains. 



The type is in the collections of the University of 

 California. 



16. Placcnticeras pacificum Smith. 



Plate VIII, Figs. 162-164 and 171-172; Plate IX, Fig. 180. 



Placeiiticer as pacific iivi Smith, Proc. Cal. Acad. Sci., 3d Ser., Geol., Vol. I, 

 pp. 207-210, Pis. XXV-XXVIII. 



Shell discoidal, involute, compressed, and moderately smooth; size of 

 largest shell about 16.5 cm. in greatest diameter. The species is related to 

 the preceding and superficially differs from it chiefly in being smoother and 

 more graceful in its ornamentation. As shown in the figures and description 

 (1. c), in its younger stages it is characterized by its smooth form, without 

 ribs or tubercles. The development of the two species is entirely different. 



1 Proc. Cal. Acad. Sci., 3d Set. Geol., Vol. I, p. 181. 



