AdAXIDES. 113 



rounded abdomen; umbilicnl slioulders l)roadly rounded, abdominal shoul- 

 ders more abrupth'. (rreatest width of wliorl at aliout onc-fittli of height 

 of whorl. Involution almost one-half of height of whorl. Undjilious very 

 narrow, almost closed, and not showing the inner coils. Surface smooth, 

 no constrictions, ribs, or other ornamentation nt maturity; but tlie writer 

 has observed constrictions on a specimen of diameter 13 mm. Septa 

 composed of a tongue-shaped ventral lobe, with a narrow, long, pointed 

 lateral lobe on each side, narrow external saddle and broad lateral saddle. 

 On the umbilicus there is a short broad lobe, and concealed by the involu- 

 tion there is a pointed spatulate antisiphonal lobe, resembling the external 

 one, and on eacli side a narrow pointed lateral lobe. The septa figured 

 here are from actual drawings from a specimen broken open to expose the 

 interior. Number of septa to a revolution from 17 to 20. 



James Hall at first considered (Touiaiites rotdtoriiis and G. ixion to be 

 identical, but afterwards changed his opinion. The differences on which he 

 based the specific discrimination were the greater number of septa to a 

 revolution in (i. ixion and the greater lateral compression. But these very 

 characters are not constant in the specimens from Rockford, the number of 

 septa to a revolution varying from 17 to 20, and some shells are more com- 

 pressed than others. The variation between individual specimens from 

 Rockford is at least as great as the differences between de Koninck's and 

 Hall's types, and most paleontologists have always believed in the identity 

 of the species. 



The following are the dimensions of a Rockford specimen: 



Millimeter. 



Diameter 68. 0=1. 00 



Height of last wliorl 39. 0= .56 



Height of last whorl from the preceding 21. 0= . 30 



Width of the last whorl 27. 0= .40 



Involution 18. 0= . 26 



Width of umbilicus 2. .5= .03 



Compared with Hall's figures and those of de Koninck tliis specimen 

 might belong to either. In fact, instead of being more compressed than the 

 Belgian form, it is rather broader. 



Hyatt" chose Goniafifes ixion Hall as the type of his genus Brancoceras, 

 but this name had been preoccupied three years before by G. kSteinmann 

 for a genus of Cretaceous ammonites. It has, however, been demonstrated 



oProc. Boston Soc. Nat. Hist., Vol. XXII, p. 326. 

 MON XLII — 02 8 



