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My references, in Genera of Fossil Cep/ialopods, to some Car- 

 boniferous nautiloids as probably members of this genus were 

 erroneous. Barrandeoceras has been referred to above as belonging 

 to the Tarphyceratidae. 



Pselioceras, mentioned also in my Genera of Fossil Cephalo- 

 pods, as another member of this family, may possibly be a genus 

 of Rineceratidae, but it does not belong here. 



The family of the true Nautilidae have been properly limited 

 farther on to Mesozoic genera. 



Rhadinoeeras* 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 cornuloi. 



Nautilus cornulus, Hall {Pal. N. P., v. Pt. ii, 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 striae 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 



*^PadX/6s^ slender. 



