1904] Affinities of Palceodiscus and Agelacrinus. 35 



nevertheless distinctly enclosed by it (fig. 6). The bilobed pore and 

 its marginal position are primitive conditions. Echinoids, I shall 

 attempt to show, must have descended from ancestors (Asteroids) 

 which possessed a single pore passing between the ambulacral plates. 

 The Echinoids more than any other Echinoderm depends for its 

 existence upon the strength of its tube feet. The bifurcation of the 

 tube foot canal by the formation of a double strand of tissue, its 

 arrangement in an arch-like manner, and its enclosure in the plate 

 would strengthen the power of the tube foot considerably. Palseodiscus 

 shows the beginnings of these characters. The inner series of these 

 plates in the cast are observed to dip downwards and distally. They 

 lie in a groove which is partially filled in with sandstone. The 



TEXT FIG. 3. Inner ambulacral plates of same ; p., plates in mouth region 

 separating from each other; sp.p., splayed out portions of plates. 



existence of both inner and outer series of plates is confirmed by 

 the series of sections taken. An examination of the specimen showed 

 that the outer series of plates have suffered much from solution, but 

 are fortunately best preserved over the ambulacral groove, probably 

 because of the greater mass of calcite in this region. In the series of 

 sections taken the outer series were visible from 1 7, showing that 

 they had a thickness of ^ mm. The inner series then appeared, and 

 persisted for J mm. The outer plates were pressed down over the 

 inner series so that little trace of an ambulacral groove appeared. It is 

 this displacement combined with solution which has in ill-preserved 

 specimens, and these are the majority, left little or no trace of a 

 distinct inner series. The evidence of the behaviour of the inner series 

 in the mouth region was quite conclusive, however, as to their 

 individuality. Here they separate out just as do the ambulacral 

 ossicles forming the mouth of the skeleton of Asteroids. This 



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