214 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



shown on PL XXIV, figs. 8 and 9; the dimensions at this 

 stage were as follows: — 



mm. 



Diameter i . 32 



Height of last whorl 0.57 



Height of last whorl from the preceding 0.41 



Width of last whorl o . 90 



Involution 0.16 



Width of umbilicus o . 58 



If it had not been said that this was a minute young shell 

 taken out of an older individual, any paleontologist would 

 refer it without hesitation to the Glyphioceratidas, and prob- 

 ably to that group designated by Haug (1898, p. 40) as Pro- 

 nannites, of the Lower Carboniferous; the only character in 

 which it differs from that group is in the two internal lateral 

 lobes, and this, in the opinion of the writer, is the result of 

 unequal acceleration, causing the introduction into the 

 glyphioceran stage of elements that belonged to the descend- 

 ants of this group. Moreover, there must have been some 

 adult form with this character, for Paralcgoceras has three 

 external lateral lobes, and must have developed out of a 

 form with glyphioceran or gastrioceran shape and the super- 

 numerary internal lobes, since the new elements usually 

 develop inside the umbilicus. This stage ends at one and 

 one-fourth coils, diameter 1.50 mm., having lasted about 

 one-half a revolution. 



Adolescent Stages. 



Ananeanic — Cymbites Stage. — When the young animal 

 has taken on characters that, if occurring in an adult, would 

 stamp it as an ammonite and not a goniatite, it may be con- 

 sidered as adolescent; by this it is not meant to imply that 

 there is any sharp line of demarcation between these two 

 groups, for there are a number of genera that might, with 

 equal propriety, be classed in either division; it is admitted, 

 too, that this manner of subdividing ontogenic stages is 

 artificial, and that it would be much more satisfactory 

 merely to call a stage by the name of the genus with which 

 it is correlated, if we could only be sure of this correlation. 

 But, on account of unequal acceleration, and obscure 



