MIDDLE CAMBRIAN. 



37 



examples. This view was strengthened by the finding of specimens in which 

 the group of oral arms formed was much smaller than the exumbrella, as 

 shown by figs, la and 2 of PL XVIII. The simplest type of these arms is 

 shown by fig. 3a (x, x, x) of PI. IX and figs, la (x, x) and 2a (x, x) of PI. 

 XVII, where the arms are rounded, not flattened like those of figs, la and 2 

 of PI. XVIII. The interior 

 canals are clearly shown 

 (x, ./', ./) in the former, and 

 the arms of la, PI. XVII, 

 appear to merge into and 

 form a part of the exum- 

 brella. This may arise from 

 the coalescing t h r o u g h 



preSSUre Of the arms and Fig. 12.~ Laotira cambria. Restoration of the canal system of the sub- 

 . iin r umhrellaof the specimen illustrated by fig. 1 of PI. XII. 



the subumbrella surface 



out to the margin of the exumbrella. The lobes or arms of all these have 

 a canal running from their termination inward toward the center, which is 

 apparently similar to that of the exumbrella lobes. This is still more 

 marked in figs. 3a, 3b, and 4a of PI. XVIII, where there is every appearance 

 of two individuals locked together. In figs. 3, 3a, 3b, and 3c of PI. XVIII, 

 the lower side is formed of simple lobes or arms; and the exumbrella is 



one of the first stages of the com- 

 plex type. After long study of 22 

 specimens showing these features in 

 a marked degree, I have 110 other 

 interpretation of them than that they 

 are infolded oral arms. This view 

 is strengthened by figs. 5 and 5a 

 of PI. X, where a relatively simple 

 complex type of exumbrella has a 

 complex subumbrella with several 

 oral arms (x, x, fig. 5a) that, considered individually, differ but little from 

 the oral arms of figs. 3 and 4 of PI. XVIII. The more complex type is 

 shown by fig. 5a of PI. XIII. The exumbrella (fig - . 5) is highly irregular, but 

 its irregularity is increased on the subumbrella side by the infolding of the 

 exumbrella lobes and the presence of the irregular oral arms. This is still 



Fig. 13.— Laotira Cambria. Theoretical diagram of the 

 canal system of the exumbrella and subumbrella lobes of the 

 specimen illustrateu by tigs. 2, 2a ol PI. XTT. 







c 



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