MIDDLE CAMBRIAN. 35 



The system of canals of the exumbrella is shown by text figs. 11 

 and 12 (pp. 36, 37). We are not dependent upon the assumption that 

 such canals exist in the complex forms, as they have been exposed by 

 weathering in numerous specimens, one of which is illustrated by fig. 5 of 

 PI. XII. I think it is fair to assume that lobes or arms of the character of 

 those illustrated in text fig. 10 are attached below the various centers of 

 text fig. 11. 



Fig. 1 of PI. XII illustrates the subumbrella surface of a specimen 

 in which the subumbrella lobes are united in three centers which can not 

 be distinguished upon the upper or exumbrella surface, owing partly to its 

 being obscured by attached material. From the number of large lobes 

 indicated on the exumbrella surface, and from analogy with other speci- 

 mens, it is probable that the two surfaces were as unlike as those of the 

 specimen represented by figs. 2 and 2a of PI. XII. The special point to 

 which attention needs to be called is the fact that nearly all, if not all, of 



o 



Fig. 10.— Laotira cambria. Diagrammatic transverse section of a complex form, such as is represented by fig. 2 of 

 PI. XIII. 



a, upper or exumbrella lobes ; b, interlobes with canal corresponding to the interlobe " & " of text fig. 6 (p. 29) ; 

 o, oral arms or lobes with interior canal; g, same as ill fig. 9. 



the lobes of the subumbrella surface of this specimen are united, as shown 

 in the figure. The presence of canals in such lobes is shown in a number 

 of specimens, and apparently we have in this an example in which the canal 

 system is confined almost entirely to the fixed portions, and there is prac- 

 tically little, if any, representation of the free oral arms or lobes so char- 

 acteristic of many other specimens. 



Figs. 2 and 2a of PI. XII illustrate a relatively simple form of exum- 

 brella with a complex subumbrella. The canal system of the exumbrella and 

 subumbrella is diagrammatically represented by text fig. 13, \x 37. The canal 

 system of the exumbrella is shown by heavy dark lines on the exumbrella 

 lobes, and the subumbrella canals are represented by dotted lines. Of the 

 canals connecting these two systems of canals we have no knowledge, as none 

 of the specimens of which transverse sections have been made illustrate this 

 feature. This is owing probably to the compression of nearly all specimens 



