2IO CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



species not especially characteristic. This fauna almost 

 undoubtedly indicates the lower part of the Chico formation 

 Cenomanian, Upper Cretaceous. 



P. pacificmn was afterwards found by Mr. F. M. Ander- 

 son one-half mile west of Henley, near Hornbrook, Siski- 

 you county, California, associated with P. calif or niaim, 

 Pachydisciis newberryanus Meek, Phylloceras ramosu?n 

 Meek, Hoplitcs cf. remondi Gabb, Chione varians Gabb, 

 Mactra ashbitrneri Gabb, Cinulia obliqna Gabb, Cylichna 

 costata Gabb, and a number of other characteristic Chico 

 species; from this locality came the type of the species, 

 figured on PI. XXVI. Mr. F. Rolfe has recently found 

 this species in the lower Chico beds of the canyon of 

 Silverado creek, near the old coal mine, about two miles 

 east of the mouth of the canyon, where Silverado empties 

 into Santiago creek. 



Placenticeras facificum, since it occurs in the same hori- 

 zon in northern, middle and southern California, may be 

 taken as characteristic of the lower Chico ; so the refer- 

 ence of the beds to the Horsetown, made by the writer 

 (Smith, 1898) in a former paper, will have to be revised, 

 for a careful study has shown the accompanying faunas at 

 all three localities to be characteristic of the Chico, and not 

 of the Horsetown. 



Larval Stages.* 



Phylembryonic Stage. — The young Placenticeras was un- 

 doubtedly shelled before it was hatched, although it was 

 not possible, on the specimens under investigation, to find out 

 certainly the limits of the primitive embryonic body-cham- 

 ber; but this seems to have coincided approximately with 

 the limits of the protoconch, although it may have included 

 somewhat more of the spiral coil. Branco (1879, P- 24), on 

 the other hand, is of the opinion that the embryonic shell 

 could not have taken up the entire protoconch, but must have 

 been homologous with the primitive cap-shaped shell of the 



'In the nomenclature of stages of growth the writer has followed Hyatt's "Phylogeuy 

 of an Acquired Characteristic." 



