MIDDLE CAMBRIAN. 23 
present. Oral plate quadripartite, with four oral arms starting out from it, 
but whether these branch or not is not known. A second type of oral arms 
may be represented by the interradial lobes. Type, Brooksella alternata. 
BROOKSELLA ALTERNATA Walcott. 
Pls. I-LY. 
Broksella alternata Walcott, 1896. Proc. U.S. Nat. Mus., Vol. XVIII (1895), p. 612, 
P], XXXI, figs. 1-5. 
The variation is so great in this species that a brief specific diagnosis 
is of little value. For the purpose of special description the external form 
and parts will first be considered and then what is known of the “ gastro- 
vascular” system. The average size of the specimens thus far collected is 
about 4°. A few individuals reach 5™, and a number occur below 2 in 
diameter, but none less than 1°. 
umbretla— The general form of the umbrella as preserved in the fossil 
state varies from subspherical to a somewhat depressed convex disk. Fol- 
lowing Haeckel, the dorsal surface will be called the exumbrella; the ven- 
tral surface, the subumbrella; the central section of the umbrella inclosing 
the stomach and oral organs, the wmbrella disk; and the peripheral section, 
or umbrella margin, the wnbrella corona. The least compressed specimen is 
illustrated by figs. 4, 4a, 4b of Pl. I. These give the impression that the 
medusa, when living and floating in the water, was nearly spherical, with 
the exception of a flattening of the dorsal pole; this, however, is probably 
misleading, as these specimens presumably represent the umbrella when 
contracted, the expanded condition being seen in such specimens as those 
shown in figs. 1 and 2 of PI. I, figs. 1, 1a of Pl. III, ete. In the latter the 
radial ribs of the subumbrella would be drawn up, but not, normally, as 
far as in fig. 3a of Pl. I. 
Exumbretla—— The form and character of the exumbrella vary, owing to (1) 
original form and (2) condition of preservation. 
1. In its original form the lobation was more or less clearly defined 
and varied. Individuals of nearly the same size have from 6 to 12 lobes; 
in some the lobation starts from the center of the umbrella, and the surface 
of the lobes is on the general surface plane (PI. I, figs. 1 and 3), while in 
others a secondary system of lobes appears from beneath the upper lobes 
and gives great irregularity to the surface. (Pl. I, figs. 7 and 8; Pl. II, 
figs. 5 and 6.) 
