MIDDLE CAMBRIAN. 31 



position. In fig. 11 (of PI. Ill) 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 amis 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 ^Yalcott, 1896. Proc. U. S. Nat. Mus., Vol. XVIII (1895), p. 613. 



Discomedusae 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 compaiing 

 the types of Brooksella, as illustrated on Pis. I and II, and the simple forms 

 of Laotira, as illustrated on Pis. V, VI, and XVIII. There is, however, a 

 strong resemblance between the oral arms of fig. 3a of PI. I and those of 

 fig. la, 2, and 3c of PI. XVIII. 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. 



