70 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



Placenticeras californicum Anderson 

 PI. 9, Figs. 16 and 17 



P. californicum Anderson, Proc. Cal. Acad. Sci., Third 

 Series Geology, vol. 2, no. 1, p. 78, pi. 8, figs. 173-7, 1902. 



Shell discoidal, compressed, narrowing regularly from um- 

 bilical margin outwards ; marked by coarse ribs which tend to 

 develop a row of tubercles near the umbilical region, and 

 two rows near the outer margin which are elongate and nar- 

 row ; ribs turning forward on leaving the umbilicus and 

 then backward about center of side of coil, then gently for- 

 ward near the peripheral angle, only about every other one of 

 these ribs extending whole way across shell, the other half 

 arising about halfway between the others from middle of side 

 and extending to peripheral angle as do the others; space be- 

 tween tubercles at periphery flattened, its width equal to 

 about one-third thickness of shell. Locality 3. L. S. J. U. 

 Pal. Coll. 



Placenticeras pacificum Smith 

 PI. 9, Figs. 18 and 19 



P. pacificum Smith, Proc. Cal. Acad. Sci., Third Series, 

 Geol., vol. 1, pp. 207-210, pis. 25-28, 1900. 



Shell discoidal, involute, compressed, and moderately 

 smooth ; under surface ornamented with sickle-shaped ribs ; 

 peripheral angle sharp and back slightly concave, giving it a 

 double-keeled appearance. Figured specimen from locality 3, 

 in L. S. J. U. Pal. Coll. Found at locality 3. Also near 

 Henley, Cal. ; in the Santa Ana Mountains, and in Arroyo del 

 Valle, Cal. This species differs from P. californicum in its 

 ornamentation; it may be the same as Placenticeras subtilis- 

 trialum Jimbo.^* 



Placenticeras sanctaemonicae, new species 

 PI. 9, Figs. 20 and 21 



This species is intermediate between P. californicum An- 

 derson and P. pacificum Smith. The external surface is orna- 

 mented by strong sickle-shaped ribs, which are separated by 



^* Palaeontologische Abhandlungen, 1894; Kotora Jimbo; p. 25, pi. 1, figs. 1 and la. 



