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. 



Pis. I-IV. 



Broksella alternata Walcott, 1896. Proc. U. S. Nat. Mus., Vol. XVIII (1895), p. 612, 

 PL 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 cm . A few individuals reach 5 cm , and a number occur below 2 cm in 

 diameter, but none less than l cm . 



umbreiia. — 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 exumbrelia ; the ven- 

 tral surface, the subumbrella; the central section of the umbrella inclosing 

 the stomach and oral organs, the umbrella disk; and the peripheral section, 

 or umbrella margin, the umbrella corona. The least compressed specimen is 

 illustrated by figs. 4, 4a, 4b of PI. 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, la of PI. Ill, etc. In the latter the 

 radial ribs of the subumbrella would be drawn up, but not, normally, as 

 far as in fig. 3a of PI. I. 



Exumbreiia. — The form and character of the exumbrelia vary, owing to (1) 

 original forru 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. (PI. I, figs. 7 and 8; PI. II, 

 figs. 5 and 6.) 



