PLATE XII. 



Fia. Page. 



1. Gnemidactis girvanensis (Schuchert) ; photograph of the apical surface 



to show the linear arrangement of the paxillae and the upper 

 surfaces of the exposed ambulacralia, X 2. — Upper Ordovician 

 (Ashgillian) ; Thraive Glen, Ayrshire. Mrs. Gray's Collection, 

 no. D. 218 . . . . . 158 



2. Ditto ; photograph of the apical surface taken at a slightly different 



angle, X 3. — Ibid. Mrs. Gray's Collection, no. D. 73. 0., 

 odontophor . . . . . .159 



3. Ditto; photograph of the oral surface to show the spoke-like arrange- 



ment of the arms, nat. size. — Ibid. Mrs. Gray's Collection, 



no. D. 218 c. . . . . . . 159 



4. Ditto ; photograph of the oral view of a portion of one arm to show 



the ornament on the adambulacralia and the ridges on the 

 ambulacralia, x 3. — Ibid. Mrs. Gray's Collection, no. D. 73 c . 157 



5. Ditto ; photograph of the extremity of an arm to show regeneration, 



X 4— Ibid. Mrs. Gray's Collection, no. D. 110 c . . 161 



