10 



FOSSIL MEDUSA. 



however, to compare the lobed margin Avith that of figs. 7, 7a of PL XVII. 

 The four oral arms of Brooksella are not known to have been subdivided 

 (see PI. II, figs. 8, 8a) ; nor is there any trace in the fossils of a system of 

 sucking pores, corresponding to the sucking frill on the axial side of the 

 arms of Archirhiza. That such sucking pores existed is highly probable. 



The transverse section of the recent Cannorhiza connexa Haeckel is of 

 value in interpreting the structure of Brooksella. The section shows the main 

 radial canals extending from the central stomach to the margin, pillar canals, 



cc. 





■?\ IS 



(Jlpji 





Fig. 2. — Archirhiza prvmordialis Haeckel. View from below; twice the natural size. The two paired arms of the 

 left half are cut away. In the center of the arm disk the regular cross of the month seam is visible, the four branches of 

 which fork at the ends of the brachial trunks, op, perradial sensory button; oi, interradial sensory button; cc, ring 

 canal. 



arm pillars, buccal stomach, oral arms, brachial canals, and sucking mouths. 

 In the fossil forms (PI. II, fig. 8b) the oral arms, brachial canals, base and pil- 

 lar canals, radial canals, and central stomach are more or less clearly shown. 

 These are all general features, but taken in connection with the character < >1 

 the umbrella, they serve to classify the Brooksellida? under the Discomedusse, 

 but not to establish their family relation with any of the living families. 



The living forms of medusae that resemble Brooksella and Laotira most 

 clearly in their habits appear to be Polvclonia and Cassiopea (pp. 6, 7). 



