218 



BRITISH PALEOZOIC ASTEROZOA. 



It is extremely difficult to get a good cast of a paxilla unless it is end on, for the 

 air inside a vertical hollow in the mould has no method of escape and so keeps the 

 rubber out (cf. p. 151). If the holes are at an angle the rubber enters more 

 readily. 



Oral Surface (Text-fig. 120). — The displacement of the apical covering, alluded 

 to above, allows one to obtain good views of the ossicles of the oral surface as seen 



A 



m. 



Text-fig. 156. — Apical view of a portion of the arm of Schuchertia wenloeki. Am., ambulacra! 



Px., paxilla. x 12. 



from above. Text-fig. 127 (p. 187) gives such a view in the region of the mouth. 

 The odontophor is almost flat, but one can see a suggestion of the Y so 

 conspicuous in Stellaster. The arrangement of the proximal ambulacralia is 

 somewhat as in the Platanasterida3 (p. 177). The first ambulacral is small. 

 Although it is displaced it is seen, like the second ambulacral, to lie on one edge. 

 Neither of these ossicles meets its pair across the median line of the arm. The 

 more - distal ambulacralia have slightly advanced in structure from those of the 

 Ordovician species (Text-fig. 120, p. ISO). The longitudinal muscle-depressions 

 are well marked, and there is a distinct beginning of a ball and socket. The inner 

 edges of the plates are slightly raised for the attachment of the dorsal transverse 



muscles. 



Measurements. — Specimen no. 490 — R : r : : 21 nun. : 6*5 nun. Specimen no. 



485 — R : r : : 20 mm. : 7 mm. 



Horizon mnl Locality. — Wenlockian (the Starfish Bed); Gutterford Burn, 



Pentland Hills. 



