PLATANASTER ORDOVICUS. 



173 



adambulacral has fallen away in the month-angle drawn (Text-fig. 116), but can 

 be seen in Text-fig. 117, which is an aboral view of another mouth-angle. The 

 clearest views of the distal extremities of the mouth-angle plates are given in Text- 

 fig. 114. Here they are deeply hollowed, and the odontophor fits across the wide 

 triangular space thus formed. The upward tilt of the adoral surface of the mouth- 



Su.A 



■m. 



Ad. Am. 



Text-fig. 115. - Cross-section through the arm of Platanaster ordovicus. Ad., adambulacral ; Am., ambulacral; 

 T.M., infero-marginal ; Su. Am., super-ambulacral. x 5. 



angle plates, as shown in the figures, suggests that the form was preserved while 

 the external interdental muscles were in a state of contraction (Text-fig. 134, D. 

 musc. %i p. 193). 



The ambulacral groove is wide and petaloid. The broad, short adambulacralia 

 are very slightly higher than the ridges of the ambulacralia. A view of the ossicles 



MP 



Tbxt-fig. 116. — Adoral view of an angle of the mouth-frame of Platanaster ordovicus. Ad. y and Ad. 2 , the first 

 two adambulacralia ; 31. P., mouth-angle plate, x 10. 



of the groove showing full details is given (Text-fig. 118). The ambulacral channel 

 is deep and has precipitous sides. The concavities for the transverse muscles are 

 well marked, suggesting that they were strong. There is a marked overlap of the 

 ambulacralia, which takes the form of a wide curved peg much as in Stenaster 

 (compare with PL I, fig. 6). The ridges across the ambulacralia are as described 

 for Schuchertia (p. 183). Slight pustular elevations are seen on some of the outer 

 extremities of the ridges of the ambulacralia. Bather has noted (91, IV, p. 123) 



