ACTIXOCERATIDiE. 
207 
and traces of a plicated membrane in the interior. The fragment 
was collected at Point Detour, Michigan, upon the horizon of the 
Niagara Formation. 
Horizon. Niagara Group (Wenlock). 
Locality. Drummond Island, Lake Huron. 
Represented by the type specimen figured by Stokes, which was 
transferred to the National Collection from the Museum of Practical 
Geology. 
Huronia distincta, Barrande. 
1840. Huronia^ sp., Stokes, Trans. Geol. Soc. ser. 2, vol. v. p, 710, 
pi. lx. f. 2. 
1860. Huronia distincta, Barrande, Syst. Sil. de la Boheme, vol. ii. 
Texte iii. 1874, p. 745, pi. ccxxxi. f. 2 (after Stokes). 
Sp. Char. This species was described by Barrande from Stokes's 
figure. The type specimen consists of four complete siphuncular 
segments and part of another ; these have been cut longitudinally. 
The centre is partly filled with chalcedony which has interfered very 
much with the structure, though the endosiphon and some of its 
canals can he obscurely made out. The rate of increase of this 
fragment is only 1 in 24. H. distincta differs from H. vertehralis., 
its nearest ally, in the greater breadth of the siphuncular segments 
in proportion to their height. Their infiated rims are also less 
prominent than those of H. vertehralis. 
Horizon. Niagara Group (Menlock). 
Locality. Drummond Island, Lake Huron. 
The Collection contains the type specimen figured by Stokes. 
Genus SACTOCERAS, Hyatt h 
Gen. Char. The peculiar character of this genus consists in the 
changes which the siphuncle passes through during the growth of 
the shell. Commencing with a nummuloidal siphuncle, like that of 
Actinoceras, it gradually loses this character and becomes cyhndrical, 
or onlji very slightly infiated, and comparatively narrow. Nor is 
this all, for the deposits upon the necks of the septa (anneaux 
obstructeurs of Barrande, — a, a, in the figure), which are so com- 
pletely developed in the earlier portions of the shell as to meet in 
the centre of the siphuncle, die out, leaving the siphuncular cavity 
empty. This remarkable diminution in the size of the siphuncle 
in Sactoceras is regarded by Prof. Hyatt (loc. cit. p. 274) as ‘-a 
1 Proc. Boston Soc. I^at. Hist. vol. xxii. 1883, p. 273. 
