Geol.— Vol. I.] SMITH— PLACENTICER AS. 221 



although in the earlier adolescent periods both species are 

 very much alike. This stage of growth is most nearly 

 related to Hoplites sflendens of the Gault, Lower Creta- 

 ceous, of Europe, but differs from it in being more com- 

 pressed laterally, in the narrower abdomen, and the 

 smoother abdominal keels, which on H. sflendeiis have 

 a row of tubercles like the adolescent stage of Placenticeras 

 pacijictim. The septa of the European species differ con- 

 siderably from those of the Californian, showing none of 

 the degeneration in the approach to Placenticeras, which 

 genus, according to Douville (1890, p. 290), is a descendant 

 of Hoplites. 



In the adolescent stage the four secondary lobes, formed 

 from the first lateral by subdivision, swing more and more 

 out of the original curve, and finally in maturity all semblance 

 of the original lobe is lost. The septa then begin to bear some 

 resemblance to Placenticeras, as does also the shape of the 

 whorl, and the lack of marked sculpture of the shell. This 

 species is now evidently on the road to becoming a Placen- 

 ticeras, although it has not yet reached that stage of devel- 

 opment; this seems to confirm the conjecture of Douville 

 as to the origin of that genus, which was formerly classed 

 with the Amaltheid^e. 



Ephebic or Adult Stage. 



When the septa have reached the characters of Placenti- 

 ceras, and undergo no further generic development, the 

 adult stage may be said to have begun. But these charac- 

 ters begin singly, so there is no sudden transition from 

 Hoplites to Placenticeras. Retardation in development of 

 the septa shows itself early in the adolescent period, and 

 grows more marked as the stage advances, so that the full 

 generic development of Hoplites is never reached. At 

 the diameter of 15 mm. the septa have already attained 

 the development of Placenticeras, while the shell is de- 

 cidedly a Hoplites. But when the shell becomes extremely 

 discoidal, the umbilical knots strong, the lateral sculpture 

 weak, and the central and marginal ventral keels a 

 decided feature, the shell is no longer comparable to any 



