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



characters, no one stage is exactly correlative with any 

 particular genus. Thus in naming the stages after genera, 

 it is merely meant that the characters of those genera are 

 predominant. This effect of unequal acceleration becomes 

 more marked as the adolescent stage advances. Near the 

 end of this stage appears the central abdominal keel of 

 Placenticeras. 



Paraneanic — Hoplites Stage. — Near the beginning of the 

 fourth coil, diameter slightly over 8 mm., the ribs are reduced 

 to mere faint undulations and fine sickle-shaped striseon the 

 sides and umbilicus, w^hile the external tubercles become 

 almost obsolete, forming mere notches on the continuous 

 abdominal keels. Specific characters begin to appear here, 

 and there is no longer any doubt as to the family to which 

 it belongs; this may be considered as the beginning of the 

 Ho^lites stage. The septa have not yet reached the com- 

 plete development of the genus, although on PL XXVIII, 

 figs. I and 2, a considerable advance over the Cosnioceras 

 stage may be seen, especially in . the digitation of the 

 secondary lobes. A perfect specimen of three and a half 

 coils, figured on PI. XXIV, figs. 20 and 21, showed the 

 transition from the cosmoceran to the Hoplites stage, and 

 gave the following dimensions: — 



mm. 



Diameter 12.00 



Height of last whorl 7 . 00 



Height of last whorl from the preceding 5 • 50 



Width of last whorl 3 . 60 



Involution i . 50 



Width of umbilicus 2 . 00 



With this stage begin the umbilical knots, which persist 

 and grow stronger, being characteristic of the mature Pla- 

 centiceras; the sculpture becomes fainter, and the tubercles 

 on the shoulder keels subside into faint notches; at the same 

 time the lateral compression becomes more pronounced, 

 and the shell becomes discoidal and very high-whorled. 

 Placenticeras -pacijictmi at this stage is wholly unlike P. cali- 

 fornicum, with which it is associated, being much more com- 

 pressed and discoidal, with narrower abdomen, flatter sides, 

 much less distinct sculpture, and narrower umbilicus, 



