36 Mr. W. K. Spencer. On the Structure and [Mar. 22, 



separation, which is quite distinct, causes the inner series to occupy 

 a position which is incompatible with the view that they are the 

 inturned edges of the outer series of the ambulacral plates. 



Traces of the aboral ambulacrum may be seen squeezed through on 

 to the ventral surface. The ambulacral plates in this region as noticed 

 by Gregory appear much narrower than those of the ventral surface* 



Outer 



TEXT FIGS. 4 and 5. Portions of Half Pyramid of Palceodiscus ferox. Fig. 4, 

 internal view, p.l.,processus labialis; l.e.p., lineceeminentes pyrami dales ; 

 c., cut away portion of pyramid. Fig. 5, external view ; c.p., concave 

 outer surface ; r., ridges on pyramid. 



In two of the inter-radii examined were portions of pyramids. It was 

 found possible to reconstruct the proximal portion of one of these. 

 The lateral surfaces still preserved the ridges which served for the 

 attachment of the interpyramidal muscles. They were concave, as 

 they are in the young Echinoid (Love"n 11), and thus present 

 another of the embryonic features so peculiar to Palseodiscus. The 

 external surface is deeply cut away. The median elevation forms a 

 well-defined "processus labialis." Towards the proximal end of the 

 exterior surface there is a notch, which is continued distally in the 

 substance of the pyramid. This I cannot find represented in recent 

 Urchins. The " Linese eminentes pyramidales " (Val.) are well defined. 

 The structure of the teeth in recent Echinoid has been described by 

 Giesbrecht. Giesbrecht, from a study of the root end, showed that 

 the wings of the tooth arose by the superposition of lamellae, which, 

 as the tooth was examined as it approximated to the mouth, became 

 intimately connected by secondary calcification. Loven (11, p. 9) 

 noticed these lamellae in the tooth of the very young Echinoid, and 

 also that these lamellar structures soon disappeared, presumably by 

 secondary calcifications. The tooth of Palseodiscus shows embryonic 



