174 



BRITISH PALEOZOIC ASTEROZOA. 



that the ridge on the flooring plates of Edrioaster bigsbyi may be broken np into 

 a row of tubercles. These are the only two instances of this character I have met 

 with among the Echinodermata. There are no pores for the passage of ampulloe. 

 The noses of the adambulacralia fit on large triangular faces at the outer edges 



117 



118 



M.R 



mi. 



Ad 



■I.M. 



0. 



<£ 



Text-fig. 117. — Enlarged view of a mouth angle of Platanastcr ordovicus. Ad. lt Ad.*, Ad. 3 , first, second and 

 third adambulacrals ; M.P., mouth-angle plate j 0., odontophor. x 8. 



Text fig. 118. — Enlarged view of a portion of the groove and neighbouring ossicles of P/atanaster ordooieut. 

 Ad., adambulacral ; Am., ambulacral ; /. M., infero-marginal. x 10. 



of the ambulacralia. Between the noses are shallow depressions for the tube-feet. 

 Each nose is continued as a sharp ridge across the adambulacral for about two-thirds 

 of the breadth of the ossicle, after which the ridge broadens and dies gradually 

 away, forming an outer triangular area which carries oblique transverse rows of 



Text-fig. 119. — Enlarged view of a portion of the groove and neighbouring ossicles of Palasteriseus devonicus. 



Ad , adambulacral ; Am., ambulacral. x 4. 



spines. Single spines are carried by the ridge right up to the point where the 

 nose fits on to the ambulacral. The occurrence of these inner spines is interesting, 

 because it shows that the linear arrangement of the ambulacralia and adambula- 

 cralia is a natural one, and not due to the collapse outwards of the adambulacralia 

 after death. If during life the adambulacralia had stood, as is usual, at more or 

 less a right angle to the ambulacralia, the walls of the groove would have been 



