SCHUCHERTTA WENLOCKI. 



21' 



smaller than the immediately succeeding ossicles, which are almost constant in size 

 for some distance along the arm. In consequence the proximal region of the 

 arm is straight, not petaloid as in Palasterina (see below, p. 225). 



The proximal infero-marginalia are broader than long, with a weakly concave 

 upper surface (Text-fig. 124, p. 183). About a third of the way down the arm 

 they become longer than broad and much more swollen. Finally they become 

 almost globular and pass over to the apical surface. Text-fig. 155 shows these 

 small, globular infero-marginalia clearly distinguishable from the apical paxillse. I 

 have made a suggestion above (p. 209) that some of the infero-marginalia may have 

 been modified for respiratory purposes. The infero-marginalia are approximately 

 equal in number and more or less alternate with the adambulacralia — characters 

 which appear to be constant in the genus. The interradial plates of the disc are 

 not arranged in a constant manner and also vary in number. They are polygonal, 

 with distinctly swollen central areas. 



r.M. 



Ad." 



Text-fig. 155.— Wash-di'awing- of the end of an arm of Schuchertia wenlocki. Ad., adambulacral ; 

 I.M., infero-marginal ; Px., paxilla displaced from apical surface, x 30. 



The madreporite is a large oval plate seen in all the three more perfect 

 specimens. It overlaps the other plates of the interradial area, suggesting, as in 

 Palasterina primseva (p. 223), that it was embedded in a skin which covered the 

 interradial ossicles. Details of the minute structure of the groove have already 

 been given (p. 182). 



Apical Surface (PI. XV, fig. 4; Text-figs. 127 and 156). — It is extremely 

 difficult to get a really good view of the apical covering, as the plates are usually 

 much displaced. The arrangement of the ossicles can only be seen in a rather 

 indistinct specimen (no. 490). Casts from this and other specimens (nos. 477, 478, 

 480, 482) show that the apical covering was composed of numerous small plates, 

 arranged in rows but not showing differentiation into radialia, adradialia and 

 supero-marginalia. Favourable views of the plates show them to be paxilliform 

 (see Text-fig. 145, p. 204, and Text-fig. 150, p. 218). Each paxilla possesses a flat 

 top, a long shaft and a spreading base. Spines can be distinguished on some 

 of the paxillae in specially good casts. 



