Geol.— Vol. I.] SMITH— PLACENTICERAS. 209 



Placefiticeras guadaloupcB Roemer (PL XXVIII, fig. 8) 

 suggests this origin for part of them. It is commonly 

 accepted that in the ammonites new elements in the septa 

 develop on the dorsal side near the umbilicus, and appear 

 on the outside only in later growth ; here in the develop- 

 ment of Placenticeras this is decidedly not the case, for the 

 three principal lobes are developed out of one that was on 

 the outside from the very beginning. 



Placenticeras facijicmn is most nearly akin to P. calif orni- 

 cum, with which it agrees in the width of the umbilicus, but 

 from which it differs in the more compressed whorl, more 

 numerous, finer ribs, and the smaller size of the knots on 

 the umbilicus. In P. pacijiciini the breadth of the last 

 whorl is one-fifth of the diameter, and in P. califoi'nicuni it 

 is one-fourth, but in the young stages the relative measure- 

 ments of the two species are very much the same, although 

 there is no danger of confusing them above the diameter of 

 four millimetres. P. -pacijictiin might also be compared 

 with P. guadalo7ipce Stoliczka (1861) (not Roemer), but 

 the Indian species is even thicker than P. calif ornictim, and 

 has a row of coarse knots on the umbilical shoulders, and 

 another near the abdominal shoulders, which would distin- 

 guish it from either Californian shell. Also, the ribs on 

 the Indian species do not show that strong sigmoidal bend 

 seen on the others. 



Horizon and Locality. — Placenticeras ^acificiini was first 

 found by Dr. L. G. Yates on the Arroyo del Valle, ranch 

 of Mrs. Isabella Jordan, Alameda county, California, about 

 eight miles southeast of Livermore. Associated with it were 

 found the following species : Baculites chicoensis Trask 

 Desmoceras hoff7nanni Gabb ; D. f sel-wyniamini Whiteaves 

 Hoplites remondi Gabb ; Holcostephanus ? sucicBnsis Meek 

 Lytoceras alamedense Smith; Lytoceras 3«/(?5? Trask ; L. cf. 

 cola Stoliczka; L. cf. timotheaniim Mayor; Placenticeras 

 californicum Anderson (MS.) ; Phylloceras ofioense Stanton: 

 P. ramostmi Meek ; Ciniilia ohliqua Gabb ; Inoceramus cf . 

 Vancouver ensis Shumard; Pectunculus veatchi Gabb; Tri- 

 gonia evansana Meek; Nucula truncata Gabb, and other 



