Geol.— Vol. I.] SMITH— GLYPHIOCERATID.^. II9 



This stage begins at 0.85 mm. diameter, three-fourths of 

 a whorl from the protoconch, and lasts with Httle change 

 except increase in size for two revolutions up to a diameter 

 of 2.25 mm,, when the transition to Glyphioceras begins. 



Plate XIV, figs. I and 2, shows a continuation of the 

 Prionoceras stage, at one and three-fourths whorls, with 

 dimensions: 



mm. 



Diameter i . 29 



Height of last coil 0.45 



Height of last coil from the preceding o. 29 



Width of last coil 0.95 



Involution o. 15 



Width of umbilicus o . 50 



On this specimen are seen two constrictions about two- 

 thirds of a revolution apart, thus making the resemblance to 

 Prionoceras very striking. At this stage are first seen the 

 cross striae of growth. A continuation of the same generic 

 stage is shown on PI. XIV, figs. 3 and 4, at one and seven- 

 eighths whorls, and dimensions: 



mm. 



Diameter i . 38 



Height of last coil 0.52 



Height of last whorl from the preceding 0.31 



Width of last whorl i . 02 



Involution o. 21 



Width of umbilicus 0.51 



The relative dimensions are nearly the same as at diam- 

 eters 0.92 mm. and 1.29 mm., but the sutures differ slightly, 

 the ventral lobe being slightly blunted, as shown on PI. XV, 

 fig. 8. On this specimen only one constriction was visible 

 at diameter of 0.85 mm. 



On PI. XIV, figs. 5 and 6 show a larger specimen still in 

 the Prionoceras stage, at two and one-eighth whorls. Di- 

 mensions: 



mm. 



Diameter i . 64 



Height of last whorl o . 60 



Height of last whorl from the preceding 0.40 



Width of last whorl i . 38 



Involution 0.20 



Width of umbilicus o . 54 



( 2 ) October 13, 1897. 



