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



of Palaeozoic Echinoids shows that various genera of these also possess 

 rhomboidal inter-ambulacral plates distally, but these rapidly become 

 polygonal. 



Palaeodiscus, with its great number of rhomboidal interambulacral 

 plates, shows in this, as in all other respects, very primitive features. 

 The plates are in several rows radiating from the initial plate in a fan- 

 shaped manner (fig. 1). It is this arrangement we must regard as 

 ancestral. The general Asteroid appearance is very suggestive. 



Both interambulacral and ambulacral plates are imbricating. The 

 imbrication obeys the general Echinoid rule, namely, the ambulacral 

 plates imbricate adorally, the interambulacral aborally. This imbrica- 

 tion also occurs in the Palaeozoic Lepidocentridse and Lepidesthidae, as 

 also in the Echinothuridae. Jackson (9, p. 237) points out that 

 imbrication, since it appears in such widely separated types, is "a 

 variation built upon independent lines." 



The anus is situated in an interambulacrum, whether on the dorsal 

 or ventral surface it is difficult to say, but certainly at a considerable dis- 

 tance from the apex (Plate 1, fig. 1 ). This is a position peculiar amongst 

 Echinoids to Palaeodiscus, and the closely related genus Echinocystites. 

 It was surrounded by a number of small plates, which gave it a very 

 asteroid appearance. Gregory (5, p. 132) has suggested that the apical 

 plates of Echinoids in general may be derived from the five valvular 

 plates which surround the anus in Echinocystites. The large number 

 of such plates in Palaeodiscus cannot be reconciled with this suggestion. 



There is no trace of any apical disc in the specimen examined. It has 

 been pointed out that Palseodiscus is always compressed in a horizontal 

 direction, and if an apical plate had existed its remains would have 

 been driven into the impression of the lantern of Aristotle and lost. If 

 the apical disc existed, it did not enclose the anal region either in this 

 form or in Echinocystites. There is no trace of a Madreporite. If we 

 judge from its position in Echinocystites, it would have been situated 

 near to the apex, and is lost for the same reason as would be any 

 apical disc. 



TEXT FIG. 2. a Outer ambulacral plate of Palceodiscus ferox ; dotted lines 

 indicate reconstructed portion put in from cast ; b.p., bilobed pore. 



That there were a double series of plates in the ambulacrum had 

 been inferred by Professor Sollas. The outer series consisted of 

 a number of pairs of long thin plates, which alternate irregularly with 

 one another over the middle line. They were pierced by bilobed 

 pores situated at the extreme proximal (mouth) end of the plate, but 



