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corresponding apertures like those of Baculites, but with less prom- 

 inent rostra or ventral crests. As the gerontic stage begins the 

 aperture changes in outline and the shell bends with a sharp curve 

 and forms a gerontic perforation, which is long and narrow. A 

 gerontic dorsal furrow appears in this bend and beyond it a gerontic 

 contact furrow also appears. In Sciponoceras the gerontic stage 

 has not been fully observed and in Diptychoceras these gerontic 

 characters begin to appear in the ephebic stage. 



The family characters are therefore simply the straight mode of 

 growth and the changes in the aperture and also probably the ten- 

 dency observed in all species of Ptychoceras to lose the rostrum in 

 old age. This can be seen in the backward or aborad inclination 

 of the costse on the gerontic arm as compared with the forward 

 curves of the same in the ephebic stage on the opposite parts of the 

 first arm. 



Sciponoceras.* 



This form apparently completes the series of which Ptychoceras 

 and Diptychoceras are obviously members. In comparing these 

 two last and in studying their development and comparing them 

 also with the young of Emperoceras I was struck by the peculiar 

 form of rostrum or low, broad, ventral crest indicated by the lines 

 of growth in the ephebic stage and the irregularities of the constric- 

 tions indicating apertures especially at the gerontic bends. The 

 apertures of the ephebic stage may have had considerable resem- 

 blances to the apertures of species of Baculites in the ephebic stage, 

 but when old age approaches the modifications attending the ten- 

 dency to bend in Ptychoceras are entirely distinct and the apertures 

 altogether different. 



These facts are nicely shown in D'Orbigny's figures of the type 

 of this genus, Sciponoceras {Baculites) baculoides, Terrains Cretaces, 

 PI. cxxxviii. This shell at first appears to represent the ephebic 

 and younger stages of a species of Ptychoceras, but on more closely 

 examining the drawing, if this be correct, it can be seen that it 

 is an outgrown or aged specimen having a gerontic stage of its own. 

 This is indicated by the partial disappearance of the costae near the 

 terminal aperture, which is also just beginning to make the first 

 gerontic bend and has an entirely different outline from the con- 

 strictions figured below on the cast of the same specimens. 



* SxtnCDv, a staff. 



