Geol— Vol. L] SMITH— PLACE NTICER AS. 219 



PI. XXIV, figs. 16 and 17, two and three-eighths coils, and 

 of the following dimensions: — 



mm. 



Diameter 4.10 



Height of last wliorl i . 83 



Height of last whorl from the preceding i . 55 



Width of last whorl i . 70 



Involution 0.28 



Width of umbilicus i . 16 



On this specimen the Nannites stage was seen on the 

 second coil, the perisphinctoid stage on the third coil up to 

 diameter 3.60 mm., two and a quarter coils, while the cos- 

 moceran stage begins just after the last constriction visible 

 on the shell, and lasts up to the end of the third coil, diam- 

 eter about 7 mm. In this stage the shell resembles a species 

 included by some paleontologists in the genus Ofpelia, 

 Aiimionites bipartitiis Zieten, as figured by Quenstedt in 

 "Ammoniten des Schwabischen Jura", PI. LXXXV; but 

 Quenstedt included that species under the Ortiati, and evi- 

 dently considered it as a Cosmoceras. 



A larger specimen in the Cosmoceras stage is figured on 

 PI. XXIV, figs. 18, 19, showing the distinct bundling of the 

 ribs on the sides, and the coarse abdominal tubercles; this 

 specimen at two and five-sixths coils showed the following 

 dimensions : — 



mm. 



Diameter 6 . 60 



Height of last coii . 315 



Height of last coil from the preceding 2 .40 



Width of last coil 2 . 07 



Involution o. 75 



Width of umbilicus , i . 46 



On this specimen the body-chamber occupied two-thirds 

 of the last coil, and was incomplete even then ; the last half 

 coil was taken up by the cosmoceran stage, and the first 

 half by the segoceran and the perisphinctoid stages. 



Just how long the cosmoceran stage lasts or when it ends 

 it is impossible to say, because the change into the next 

 stage is so gradual, and because it comes at such different 

 sizes on different individuals. Since the characters of any 

 one generic stage do not appear or disappear all at once, on 

 account of unequal acceleration of development of these 



