On the Structure and Affinities of Palaeodiscus and Agelacrinus . egularly withone another over the middle line. They were pierced by bilobedpores situated at the extreme proximal (mouth) end of the plate, but 1904.] Affinities of Falceodiscics and Agelacrinus, 35 nevertheless distinctly enclosed by it (fig. 6). The bilobed pore andits marginal position are primitive conditions. Echinoids, I shallattempt to show, must have descended from ancestors (Asteroids)which possessed a single pore passing between the ambulacral Echinoids more than any other Echinoderm depends for itsexistence
On the Structure and Affinities of Palaeodiscus and Agelacrinus . egularly withone another over the middle line. They were pierced by bilobedpores situated at the extreme proximal (mouth) end of the plate, but 1904.] Affinities of Falceodiscics and Agelacrinus, 35 nevertheless distinctly enclosed by it (fig. 6). The bilobed pore andits marginal position are primitive conditions. Echinoids, I shallattempt to show, must have descended from ancestors (Asteroids)which possessed a single pore passing between the ambulacral Echinoids more than any other Echinoderm depends for itsexistence upon the strength of its tube feet. The bifurcation of thetube foot canal by the formation of a double strand of tissue, itsarrangement in an arch-like manner, and its enclosure in the platewould strengthen the power of the tube foot considerably. Palseodiscusshows the beginnings of these characters. The inner series of theseplates in the cast are observed to dip downwards and distally. Theylie in a groove which is partially filled in with sandstone. The spj^,. Text Fig-. 3.—Inner ambulacral plates of same ; p., plates in mouth regionseparating from each other; splayed out portions of plates. existence of both inner and outer series of plates is confirmed bythe series of sections taken. An examination, of the specimen showedthat the outer series of -plates have suffered much from solution, butare fortunately best preserved over the ambulacral groove, probablybecause of the greater mass of calcite in this region. In the series ofsections taken the outer series were visible from 1—7, showing thatthey had a thickness of /^ mm. The inner series then appeared, andpersisted for ^ mm. The outer plates were pressed down over theinner series so that little trace of an ambulacral groove appeared. It isthis displacement combined with solution which has in ill-preservedspecimens, and these are the majority, left little or no trace of adistinct inner series. The evidence of the
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectproceed, bookyear1904
