PLATE X. 



Fio. Page. 



1. Urasterella thraivensis, n. sp. ; photograph of the oral surface of a 



portion of an arm, x 4. — Upper Ordovician (Ashgillian) ; Thraive 

 Glen, Ayrshire. Mrs. Gray's Collection, no. D. 35 . . ] 38 



2. Urasterella gutterfordensis, n. sp. ; photograph of the apical surface, 



X 2. — Middle Silurian (Wenlock) ; Gutterford Burn, Pentland 

 Hills. Royal Scottish Museum, no. 93 . . 144 



3. Photograph of the oral surface of the counterpart of the previous 



specimen, x 4. Royal Scottish Museum, no. 67 . .145 



4. Urasterella ruthveni (Forbes) ; photograph of the oral surface, X 2. — 



Upper Silurian (Ludlow) ; Bannisdale Slates, High Thorns, Under- 

 barrow. Sedgwick Museum, Cambridge, no. a. 920 . . 141 



5. Ditto ; photograph of oral surface, X 4. — Ibid. Sedgwick Museum, 



Cambridge, no. a. 510 . . . .141 



6. Ditto ; photograph of the oral surface, x 2. — Upper Silurian (Ludlow) ; 



N. End of Benson Knot. Sedgwick Museum, Cambridge . 141 



