232 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



EXPLANATION OF PLATE XXIV. 



Placenticeras pacificuni, sp. nov. 



Figs. I, 2. Protoconch from front and from above; 20 times enlarged. Ar- 

 royo del VaI16, Alameda County, California. 



Fig. 3. Larval coil, diameter 1.16 mm., showing phylembryonic proto- 



conch in the centre; 20 times enlarged. Arroyo del Vall6, 

 Alameda County, California. 



Figs. 4, 5. Transition from phylembryonic to ana- to meta-nepionic stages, 

 diameter 0.76 mm., protoconch and one-third coil; 20 times 

 enlarged. Arroyo del Vall6, Alameda County, California. 



Figs. 6, 7. Protoconch and three-quarters of a coil, diameter 0.98 mm., 

 beginning of the paranepionic stage; 20 times enlarged. Ar- 

 royo del Vall^, Alameda County, California. 



Figs. 8, 9. Paranepionic, transition from glyphioceran to Namiites, one and 

 one-twelfth coils; 20 times enlarged. Arroyo del Vall6, Ala- 

 meda County, California. 



Figs. 10, II. Early adolescent (ananeanic), Cymbites stage, diameter 1.99 

 mm., one and seven-twelfths coils; 13 times enlarged. Henley, 

 Siskiyou County, California. 



Figs. 12, 13. Ananeanic, aegoceran stage, one and seven-eighths coils, diam- 

 eter 2.64 mm.; 10 times enlarged. Arroyo del Vall^, Alameda 

 County, California. 



Figs. 14, 15. Ananeanic, perisphinctoid stage, diameter 3.50 mm., two and a 

 quarter coils; 10 times enlarged. Arroyo del Vall6, Alameda 

 County, California. 



Figs. 16, 17. Metaneanic, transition from perisphinctoid to the cosmoceran 

 stage, diameter 4.10 mm., two and three-eighths coils; 10 

 times enlarged. Arroyo del Vall^, Alameda County, California. 



Figs. 18, 19. Metaneanic, cosmoceran stage, diameter 6.60 mm., two and 

 three-quarters coils; 7 times enlarged. Arroyo del ValM, 

 Alameda County, California. 



Figs. 20, 21. Beginning of paraneanic stage, transition from cosmoceran to 

 the Hoplites stage, diameter 12 mm., three and a half coils; 2.7 

 times enlarged. Henley, Siskiyou County, California. 



