Geol— Vol. I.] SMITH— PLACENTICERAS. 223 



coarser, with deeper furrows separating them, crossing even 

 the flattened abdomen. The species now begins to have 

 considerable resemblance to P. placenta, although not more 

 than ever}'- member of a genus ought to show to the type, 

 but it never departs so far from the Hoplites group as does 

 P. ■placenta. The characters of extreme maturity begin at 

 about five and two-thirds coils, diameter 118 mm; how long 

 they lasted, or whether there was any further change it is 

 impossible to say, as the largest specimen seen had diam- 

 eter 172 mm,, and still showed no senile degeneration, 

 unless the widening of the umbilicus is due to deficiency of 

 growth force. This specimen is the type of the species 

 (PL XXVI) ; it is the property of Mr. F. M. Anderson, of 

 Yreka, California. 



The genus Sfhenodiscus was included by Meek as a sub- 

 genus under Placenticeras, and has been considered by 

 Douville as having a common origin with that group in 

 Hoplites; both genera seem to have had their origin during 

 the upper part of the Lower Cretaceous, but Sphcnodiscus 

 does not seem to have been either ancestor or descendant 

 of Placenticeras, and, therefore, cannot be considered as a 

 subgenus under it. 



Placenticeras shows retardation and degeneration, but 

 Sfhenodisctis has departed still further from the parent 

 type ; its secondary lobes have swung more nearly into a 

 straight line, and have become simpler, by retardation, until 

 they are almost ceratitic in character. But this simplification 

 of the septa can not be referred to reversion to any ances- 

 tral characters, since they were never present in any of its 

 ancestors. The ontogeny of this genus is unknown, but it 

 will probably show the larval and earliest adolescent stages 

 normal in number and character of lobes, and with this 

 subdivision into secondary lobes pushed by acceleration of 

 development to an early stage. The life-history of Spheno 

 discus would then repeat a part of the ontogeny of Iloplitcs, 

 but can not give the complete record. 



