Geol— Vol. I.] SMITH— PLACENTICERAS. 203 



Placenticeras calif ornicum Aiiderson (MS.). 



Plate XXV, Figs. 1-8; Plate XXVIII, Fig. 6. 



Placenticeras californicum is most nearly related to P. guadalonpcs (Sto- 

 liczka, not Roemer), as described and figured by F. Stoliczka {1861), although 

 it is more compressed laterally, and somewhat rougher shelled than the 

 Indian species. The largest specimen found had the following dimensions: 



mm. 



Diameter 1 20 . 00 



Height of last coil 58.00 



Height of last coil from the preceding 45 00 



Width of last coil 3° • 50 



Involution 13 ■ 00 



Width of umbilicus 23 . 00 



This specimen had about si.x revolutions, showed the body chamber to be 

 about two-thirds of the last whorl, and since there was no further progressive 

 development, seems to have been fully mature. The abdomen is narrow, 

 flattened, with low central keel, and with marginal keels made up of a row 

 of elongated knots. The sides are ornamented with rough sigmoidal ribs, 

 branching out from coarse umbilical knots, and forming the knots on the 

 abdominal shoulder keels. 



Another specimen of five and a half revolutions was 

 exactly like the larger one, and gave the following 

 dimensions: — 



mm. 



Diameter 77 . 00 



Height of last coil 37 • 50 



Height of last coil from the preceding 27 .00 



Width of last coil 20.00 



Involution 10 . 50 



Width of umbilicus 13.00 



Horizon and Localitv. — A single specimen of this species 

 was found by Dr. L. G. Yates on the ranch of Mrs. Isabella 

 Jordan, on the Arroyo del Valle, Alameda county, Califor- 

 nia, about eight miles southeast of Livermore. Associated 

 with it were found the following species : Bacidites chico'en- 

 52*5 Trask; Desnioceras hoffman^ii GTsMh; D. ? selwymmitim 

 Whiteaves ; Hoplttes remondi Gabb ; Holcostcfhamis ? suci- 

 aensis Meek; Lytoceras alamedense Smith; Lytoceras batesi 

 Trask; L. ( Tetragonites) zi. cola Stoliczka; L. cf. timothe- 

 anum Mayor; Placenticeras pacijlcum, sp. nov.. Smith; 

 Phylloceras ono'ense Stanton ; P. ramosiini Meek ; Cimilia 

 ohliqua Gabb; Inoceramus cf. vancouverensis Shumard; 



