MIDDLE CAMBRIAN. 



29 



Fio. 5.— Brooksella alterna- 

 ta. Diagrammatic central 

 vertical section through the 

 central axis between the radi- 

 ating lobes. 



The description of the let- 

 tering is to be found under 

 text fig. 4 (p. 28), with the ex- 

 ception of that of the letter I, 

 which refers to that port ion of 

 the umbrella between the free 

 lobes and exterior to the pil- 

 lar canals. 



In the form under consideration the coronal intestine is confined, so 

 far as we know at present, to the simple, direct, radial canals extending 

 from the central disk to the margin of each exumbrella „ 



and interumbrella lobe. These canals are large in pro- 

 portion to the size of the lobes (see PI. II, figs. 6 and 7; 

 PI. Ill, figs. 3 and 4; PI. IV, figs. 7, 8, 9, 10a, 11a, 13) 

 and are not branched. There is no trace of a connect- 

 ing coronal intestine, and it is probable that the canals 

 terminated in a minute excreting opening at the margin 

 of the exumbrella, as similar canals do in Aurelia, 

 although in the latter a coronal canal exists. 



Genitalia. — No traces of the reproductive system have 

 been observed. 



oral opening, piate, and arms. — No traces of an oral opening 

 have been seen. In a number of specimens, where the 

 oral plate and arms have been re- 

 moved prior to the fossilization of 

 the animal, a circular depression is 

 seen at the base of the central axis (PI. I, fig. 3a, #'). 

 This was probably the location of the mouth at an 

 early stage in the evolution of this species and the 

 development of the individual, but with the develop- 

 ment of the oral plate and arms it was covered over. 



The presence of a typical oral 

 plate and arms was for a long 

 time in doubt; upwards of 1,500 

 specimens had been collected 

 before one was found in which they were preserved, 

 and this specimen is thus far unique (PI. II, figs. 8, 

 8a, 8b). The medusa was crushed obliquely down 

 and the exumbrella lobes turned inward, the oral plate 

 and arms being pushed to one side of the center. 



The oral plate is formed by the union of the four 

 arms at the center (PI. II, figs. 8, 8a). Each arm 

 is broken off about l cm from the center; the sections of the terminal por- 

 tions have an average diameter of 5 mm by a depth of 7 mm ; the opening of 



Fig. 6. — Brooksella alternata. 

 Diagrammatic vertical section 

 after tigs. 4, 4a, 4b of PI. I, so as 

 to cut between the subumbrella 

 lobes. Interior restored. 



a, exumbrella lobes, with ra- 

 dial canals; b, interlobe, with 

 interior canal ; A\ axial or pillar 

 canals; o, oral arm, with inte- 

 rior canal ; x t central axis. 



Fig. 7. — Brooksella alternata. 

 Diagrammatic vertical median 

 section of same specimen as text 

 fig. 6, soas to cut through the sub- 

 umbrella and exumbrella lobes 

 and the central axis. The radial 

 canals and central stomach (or in- 

 testinal tube) are drawn in from 

 information partly obtained from 

 other specimens. 



