530 
My references, in Genera of Fossil Cephalopods, to some Car- 
boniferous nautiloids as probably members of this genus were 
erroneous. Barrandeoceras has been referred to above as belonging 
to the Tarphyceratide. 
Pselioceras, mentioned also in my Genera of Fossil Cephalo- 
pods, as another member of this family, may possibly be a genus 
of Rineceratidz, but it does not belong here. 
The family of the true Nautilide have beén properly lmited 
farther on to Mesozoic genera. 
Rhadinoceras,* n. g. 
The species here noticed under this name were formerly included 
in the genus Nephriticeras. They have compressed elliptical or 
almost rounded whorls, growing more slowly than in Nephriticeras, 
_ have the impressed zone only in the later stages of growth and are 
transitional between gyroceran forms and Nephriticeras. 
RHADINOCERAS CORNULUM. 
Naum:EUS CORNULUS,,Hall (Pal. 2. ¥., vo Pion, Pl lx): 
Hall’s figure shows the nepionic and neanic stages of this shell, 
and there is a slight contact furrow. 
The form of the whorl in section is almost circular, not changing 
much throughout the nepionic stage. 
The sutures are similar to those of Nephriticeras, with slight 
ventral and dorsal lobes and rather narrow lateral lobes. 
The siphuncle, according to Hall, is dorsad of the centre. 
The shell has only fine strize and fine longitudinal ridges. 
Having studied the original of this species in Prof. Hall’s collec- 
tions, I can confirm his observation and state that this is obviously 
a close-coiled nautilian form with a slight contact furrow produced 
after the whorls come into contact in the ananeanic substage, but 
- not existing previously. 
The umbilical perforation was very large, and young shells show 
that Rhadinoceras contains transitional forms between Nephriti- 
ceras and some cyrtoceran ancestor. In other words, these two 
genera were not derived from any coiled nautilian form of the 
Devonian or Silurian, but are progressive modifications of some 
closely allied arcuate form. ‘This conclusion is sustained also by 
the existence of a peculiar cyrtoceran form associated with these 
ty "Padivog, slender. 
