MIDDLE CAMBRIAN. ll 
position. In fig. 11 (of Pl. IIT) few are seen, but in figs. 12, 12a, 12b, and 
13 they are numerous. In fig: 13 they extend down so far on the central 
axis that they appear one on the other in a confused pile. It seems to be 
owing to the great development of the more or less irregular interlobes that 
the peculiar arrangement of the ventral surface is produced. 
The gastrovascular system of the umbrella, so far as known, does not 
differ from that of B. alternata, but the oral openings and arms must vary 
materially There does not appear to be a true central oral opening; and 
a careful study of the specimens leads to the view that the hollow, free 
interlobes and basal lobes or arms served as the oral arms and conveyed 
food direct to the intestine or stomach in the central axis. If this view is 
correct, the free interlobes of B. alternata served the same purpose in addi- 
tion to the true oral arms of that species. 
Genus LAOTIRA Walcott. 
Laotira Walcott, 1896. Proc. U.S. Nat. Mus., Vol. XVIII (1895), p. 613. 
Discomedusze with a lobate umbrella, 4, 5, 6, 7, to 12 or more lobes in 
the simple forms, and with a large number in the compound forms; without 
tentacles and without central oral openings in the adult; with a simple 
radial canal in each lobe of the umbrella and in the interradial lobes 
attached to the central axes, when the latter are present; oral arms repre- 
sented by interradial lobes attached to the central axis and to the subum- 
brella lobes; reproduction sexual or by fission. Type, Laotira cambria. 
The genus Laotira differs from Brooksella very strongly in most of the 
specimens. In the simple forms it approaches Brooksella in general appear- 
ance, but there is a fairly constant variation, as may be seen by comparing 
the types of Brooksella, as illustrated on Pls. I and II, and the simple forms 
of Laotira, as illustrated on Pls. V, VI, and XVIII. There is, however, a 
strong resemblance between the oral arms of fig. 3a of Pl. I and those of 
fig. la, 2, and 3e of Pl. XVII. The complex forms of Laotira differ widely 
from any known forms of Brooksella, with the exception of the subumbrella 
surface of Brooksella confusa. This species, when viewed from below, 
appears to be an intermediate form between the two genera. Its exum- 
brella surface, however, is so clearly of the type of Brooksella that I have 
referred it to the latter genus. 
