38 FOSSIL MEDUSJE. 



more finely exhibited by fig. 6 of PL XIII, and the more flattened individual 

 represented by fig. 5 of PL XII. The lower surface of fig. 1 of PL XIII 

 is, unfortunately, not sufficiently well preserved to be illustrated. It was a 

 combination of the characters seen in figs. 5 and 6a of PL XII. Fig. 1 of 

 PL XXI is a portion of the lower surface of a large complex individual 

 of the type represented by fig. 6 of PL XII. The variety of form of the 

 complex type is almost endless. This is owing to the original variation 

 and to the incidents of the embedding and preservation of organic remains 

 so liable to be distorted by pressure and their own weight. 



The presence of axial oral arms, such as occur in Brooksella altemata, 

 is of rare occurrence in Laotira Cambria. It is fairly well shown in figs. 3 

 and 3a of PL VIII, where several arms project out between the exum- 

 brella and subumbrella. 



oral piate. — The oral plate may be formed in the simple types by the 

 union of the oral arms at the center (as in Brooksella), but in the complex 

 forms no true plate can be said to exist. 



circulation. — The circulation of the complex forms may have been by the 

 intaking of food through the oral pores or tubes of the various irregular, 

 individual, or combined oral lobes of the lower surface of the organism, 

 followed by its passage through the irregular interior canals and final 

 expulsion through the exterior openings of the exumbrella canals. Such 

 would be anticipated to be the case if we should begin with the simple, 

 regular forms, and trace the structure as it gradually becomes more and 

 more complicated. 



Reproduction. — No traces of the reproductive system have been observed 

 in Brooksella, and none in Laotira except what appears to be a process of 

 fission. No true sexual organs have been recognized. Reproduction by 

 fission is indicated by the specimens represented on PL XIX, figs. 2 and 

 3. In these, two otherwise entire, simple individuals are held together by a 

 single lobe, the severing of which would set them free. Fig. 1 of that plate 

 illustrates a more complete union of two, and fig. 1 of PL XIII of three or 

 more. In fig. 6 of PL XII there is a general appearance of looseness that 

 might give rise to the view that a portion would have soon separated as a 

 distinct individual. The same is also true of the double form, shown by 

 fig. 3 of PL XII. 



Reproduction by lateral fission is of rare occurrence among the recent 

 medusas. Kolliker describes an instance among the Discomedusse — in the 



