208 
XAUTILOIDEA. 
degradational senile shrinkage ” ; Barrande, on the other hand, con- 
sidered it as indicating a reversion to the more simple, cylindrical 
structure of the siphuncle in the Orthoceratidae. 
The species selected by Hyatt as the type of his genus is the 
OrtJioceras Ricliteri of Barrande h 
Fig. 32. 
Longitudinal section of Sactoceras docens, Barrande, sp. (Bull. Soc. Gr^ol. de 
France, ser. 2, tom. xii. p. 453, pi. xii. A. f. 2). 
Sactoceras docens, Barrande, sp. 
1855. Orthoceras docens, Barrande, Bull. Soc. Geol. de France, ser. 2, 
tom. xii. p. 453, pi. xii. A. ff. 2, 3. 
1868. Orthoceras docens, Barrande, Syst. Sil. de la Boheme, vol. ii. 
Texte iii. 1874, p. 632, pi. ccl. 
^ Syst. Sil. de la Boheme, vol. ii. Texte iii. 1874, p. 570, pi. cccxyiii., 
pi. cccxxii., pi. cccxxiii., pi. cccxlix. 
