172 



BRITISH PALEOZOIC ASTEROZOA. 



of the arm in both a proximal and distal direction. The first two infero-marginalia 

 are incorporated in the disc. In front of these is a prominent breast-plate-shaped 

 odontophor. The month-angle plates are very large and extend backwards as far 

 as the third adarnbulacral. The cross-section of the arm, given as Text-fig. 115, 



0. 



MP. 



Text-fig. 114. — Slightly reconstructed drawing of the oral view of an arm and a portion of the disc of 

 Platanaster ordovicus. Ad., adarnbulacral ; Am., ambulacral; I.M., infero-marginal ; M.I'., mouth-angle 

 plate ; O., odontophor. x 3. 



shows the shallowness of the groove, the great depth of the ambulacral channel, 

 and the apical covering of uniformly sized paxilla?. 



Oral Surface (Text-figs. 116— Hi)).— The mouth-angle plates are high, narrow 

 wedges which project well beyond the proximal limits of the groove into the 

 mouth-cavity. The proximal extremity of each plate rises steeply and carries a 

 row of spine-pits along its adoral edge. Stout spines, which once fitted on these, 

 still remain scattered between the plates. The slightly swollen inner surface is 

 excavated for the attachment of the first three adambulaeralia. The third 



