MIDDLE CAMBRIAN. 34) 
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. XVII. The simplest type of these arms is 
shown by fig. 3a (a, x, x) of Pl. IX and figs. la (x, x) and 2a (a, #) of PI. 
XVUH, where the arms are rounded, not flattened like those of figs. la and 2 
of Pl. XVIII. The interior 
canals are clearly shown 
(x, x, #) in the former, and 
the arms of la, Pl. XVII, 
appear to merge into and 
form a part of the exum- 
brella. This may arise from 
the coalescing through 
pressure of the arms and fre. 12— Laotira cambria. Restoration of the canal system of the sub- 
io bo ee eee umbrella of the specimen illustrated by fig. 1 of Pl. XII. 
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 Pl. XVIII, where there is every appearance 
of two individuals locked together. In figs. 3, 3a, 3b, and 3¢ of Pl. 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 no other 
interpretation of them than that they 
are infolded oral arms. This view 
is strengthened by figs. 5 and 5a 
of Pl. X, where a relatively simple 
Fic. 13.—Laotira cambria. Theoretical diagram of the f 
canal system of the exumbrella and subumbrelia lobes of the COMplex type of exumbrella has a 
ae hee Peace ay complex subumbrella with several 
oral arms (a, 2, fig. 5a) that, considered individually, differ but little from 
the oral arms of figs. 3 and 4 of Pl. XVIII. The more complex type is 
shown by fig. 5a of Pl. 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 
