30 FOSSIL MEDUSA. 



the brachial canal is plainly shown in each, and in a vertical cross-section 

 (PI. II, fig. 8b) the canals of two of the arms arch upward and then gently 

 downward, meeting in the center; at the highest point of each the base 

 of the pillar canals (c c) is still preserved. The size of the canals, and also 

 of the arms, has probably been enlarged during the mineralization of the 

 original specimen. The upper surface of the oral arms and plate and the 

 lobes of the subumbrella have been so closely pressed together that there 

 is not any distinct line of demarcation between them. A difference in the 

 color of the rock suggests the presence of a central stomach that has been 

 pressed out laterally, but, unfortunately, is not sufficiently well defined to 

 prove its character, or that it is what it appears to represent. 



The free interradial lobes or arms, attached to the central axis beneath 

 or between the umbrella lobes, may have served the purpose of oral arms. 

 This appears to have been the case in Brooksella confusa, where there is no 

 evidence of the presence of oral arms of the Discomedusse type. 



surface. — What may have been the surface of the exumbrella and sub- 

 umbrella of a large number of specimens has a slight rugosity given to it 

 by semi-inosculating, irregular, raised and depressed lines extending from 

 the center toward the margin (PI. I, fig. 1). This occurs usually in the more 

 perfect specimens; many others do not show it, but in these there is reason 

 to think that the original surface has been worn off, or that it is obscured 

 by a siliceous deposit. 



Brooksella confusa Walcott. 

 PI. Ill, figs. 11, 12, 12a, 12b, 13. 



Brooksella confusa Walcott, 1896. Proc. U. S. Nat. Mus., Vol. XVIII (1895), pp. 612- 

 613, PI. XXXI, figs. 7, la, lb. 



In the external form and appearance of the exumbrella this species is 

 similar to B. alternata, but it differs materially in the arrangement of the 

 lobes of the subumbrella. This is shown by comparing figs. 3a, 8a, and 9a 

 of PI. I and fig. la of PI. II (the subumbrella surface of B. alternata) with 

 figs. 11, 12b, and 13 of PI. Ill (the same surface of B. confusa). In the 

 former the lobes unite at the center and preserve the true radiate structure 

 from a central axis. In the latter (B. confusa) the lobes do not join at the 

 center, but have a tendency to form an irregular union near that point 

 (fig. 13), or a double center (figs. 11, 12b). 



The interumbrella lobes of B. confusa vary greatly in number and 



