110 CARBONIFEROUS AMMONOIDS OF AMERICA. 



The smallest specimen seen (PI. XXI, figs. 10a and 10b) bad a diameter 

 of 5.5 mm., was evolute, broad and low-whorled, with caronate form, 

 strong umbilical ribs, and ^Jeriodic constrictions. This unmistakably 

 corresponded to Gastrioceras. At one-half revolution in addition to the 

 above the dimensions were: 



Millimeters. 



Diameter 7. 75 



Height iif lai^t whorl 3. 00 



Height of last whorl from the preceding 2. 25 



Width of last whorl 5.00 



AVidth of umbilicus 2. 75 



Involution 0. 75 



The gastrioceran ribs still persisted, but the whorl had become more 

 highly arched, and the septa were transitional from Gastrioceras to Paral- 

 ec/oceras, while the form was still typical of Gastrioceras. The addition of 

 another half coil showed no change in the shell, except that the ribs were 

 seen only on the umbilicus. At this stage the dimensions were: 



Millimeters. 



Diameter 10. 5 



Height of last whorl ,- 3-5 



Height of last whorl from the preceding 2.5 



Width of last whorl 6.0 



Width of umbilicus -i- 



Involution 1-0 



This is shown on PI. XXI, figs. 11a, lib, and lie. 



Another revolution showed the whorl highly arched and helmet-shaped, 

 and the ribs had disappeared ; the resemblance to Paralegoceras was now 

 plain, but the septa showed an extra pair of lateral lobes just within the 

 umbilical border. No species of Faralegoceras has yet been observed with 

 this extra pair of lobes, but such may yet be found, since only a few 

 specimens of that genus are known. This stage (shown on PL XX, iigs. 1 

 and 2, and PI. XXI, fig. 12) gave the following dimensions: 



Millimeters. 



Diameter 21 



Height of last whorl 10 



Height of last whorl from the preceding 7 



Width of last whorl 12.5 



Width of umbilicus 6 



Involution 3 



At diameter of 30 mm., one-half revolution more than the preceding, 

 tlie shell had taken on mature characters, the septa were typical of Schisto- 

 ceras, and after this only an increase in size took place. This early mature 

 stage is shown on PI. XX, fig. 3. 



