Geol.— Vol. L] SMITH— PLACENTICERAS. 1 95 



nistis d'Orb. belongs to Ofpelia, and that there are several 

 series of these plate-shaped ammonites, commonly ascribed 

 to the Amaltheidas, but in reality derived from wholly differ- 

 ent groups. 



The true species of Placenticeras, that is of the type of 

 P. placenta, are descendants of Hoplites, and thus have 

 no kinship with the Amaltheidse; this is the opinion of 

 H. Douville, and it has been fully confirmed by an ontogenic 

 study of two species. These species had been under inves- 

 tigation by the writer for a long time before their real affin- 

 ities were found out; the largest specimens obtained were 

 between 20 and 30 millimetres in diameter, and were natu- 

 rally supposed to be adults, for on the last coil there was no 

 further progressive development. One species was remark- 

 ably like Hoflites dufrenoyi, and the other like H. splen- 

 dens, both of the Gault; thus they were assigned to Ho-p- 

 lites, although the premonition of Placenticeras was shown 

 in the provisional name, " H. placenticeratoides,^'' given by 

 the writer, and in the unpublished description, in which it 

 Avas said that this species was tending decidedly toward 

 becoming a Placenticeras, although it had not yet quite 

 developed beyond the limits of Hoplites. After this was 

 written, although luckily not published, the writer obtained 

 a number of larger specimens of both species, up to 170 

 millimetres in diameter, showing a perfect and gradual 

 transition between the two genera. 



The earliest known species of Placenticeras occur along 

 with Sphenodiscus in the zone of Mortoniceras rostratuni 

 ( = S chloenbachia injlata), in the Albian stage, upper Gault, 

 top of the Lower Cretaceous. Thus the genus must have 

 developed out of Hoplites in the upper part of the Lower 

 Cretaceous. Now since the origin and affinities of Hoplites 

 are pretty well known, a discussion of this genus becomes 

 of prime importance in an investigation of the derivation and 

 systematic position of Placenticeras. 



The genus Hoplites was estabhshed by Neumayr (1875) 

 to include forms with rather compressed flattened sides, 

 high whorls, moderately narrow umbilici, sinuous lateral 

 ribs with umbilical and marginal knots or spines, and usually 



