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



species of Ho-plites, even though many characters of that 

 genus still remain. This may be called the transition to 

 Placenticeras. 



A specimen with the transition just beginning is figured 

 on PL XXV, figs. 9 and 10, four coils, and diameter 20.5 

 mm., showing the fine sculpture, sharp, sfightly notched 

 marginal keels, and narrow umbilicus with faint nodes. 

 A cross-section of this stage is shown on PL XXVIII, fig. 5. 

 This stage continues unchanged to four and five-eighths 

 coils, diameter of about 38 mm., where the ribs become 

 suddenly coarse, and undulating nodes begin to show on 

 the marginal keels. A specimen of this stage is figured on 

 PL XXV, fig. II, showing the sudden transition from 

 fine to coarse sculpture. Now, for the first time, this 

 species can certainly be placed among the typical members of 

 Placenticeras^ as defined by Meek, so the transition period is 

 over, and the animal is really mature in characters, although 

 not yet in size, since it grows nearly five times as large, and 

 adds many more specific characters. This stage continues 

 unchanged for nearly a revolution, the septa growing some- 

 what more complex, but advancing little beyond those seen 

 at diameter of 15 mm. The septa, at the latest stage where 

 they were visible, five and a-third coils, are shown on PL 

 XXVIII, fig. 4. The septa of P. giiadahtipcB Roemer, 

 copied on PL XXVIII, fig. 8, show a decided resemblance to 

 those of P. pacijicuni, but are more finely digitate, and have 

 two more auxiliary lobes; but the arrangement of the three 

 principal lateral lobes in a backward-pointing curve is sug- 

 gestive of a similar secondary origin. Placenticeras placenta 

 Dekay has departed still further from the parent type and has 

 its lobes arranged almost in a straight line, and this character 

 is pushed back by acceleration of development until it is seen 

 even in the larval stages, as shown by Jackson (1899, PL 

 XXV, figs. 118 and 119). The early larval stages w^ere not 

 figured, so it is not possible to be sure that the lateral lobes 

 developed by subdivision, but this is very probably the case. 



In extreme maturity the central abdominal keel becomes 

 obsolete, and the fine notches on the marginal keels are 

 transformed into rough elongated nodes ; the ribs become 



