PLATE VIII. 



Fig. Pagb. 



1. Urasterella thraivensis, n. sp. ; photograph of a model of a small portion 



of the ambulacral groove, X 10. — Upper Ordovician (Ashgillian) ; 

 Thraive Glen, Ayrshire. A key to this photograph is given, Text- 

 fig. 83, p. 132 . . . . . . 131 



2. Photograph of the right half of the same model taken in a direction 



facing the wall of the groove. The ossicles are seen tipping 

 towards the mouth, as also does the muscle depression on the outer 

 extremity of each ambulacral . . . .132 



3. Photograph of ossicles of the same model taken so as to show the 



ambulacral groove in vertical cross-section . . .133 



4. Urasterella medusa, Hudson ; photograph of the mouth-region in apical 



view, X 10 (from a photograph supplied by Hudson) . . 135 



5. Gnemidaster girvanensis (Schuchert) ; photograph of a model of the 



mouth-region in apical view, X 7 . . .135 



G. Photograph of the same model showing the oral surface as seen from 

 within the mouth. A key to this photograph is given, Text-fig. 103, 

 p. 160 . . . . . . 135 



7. Urasterella medusa, Hudson ; photograph of portion of the apical surface 

 of two arms to show the paxilliform infero-marginalia, X 20 (after 

 Hudson). Pores for papulae (?) are also seen on the inner side of 

 the infero-marginals 2 and 3 on the right arm . . .130 



