On the Structure and Affinities of Palaeodiscus and Agelacrinus . 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 behaviour of the inner seriesin the mouth region was quite conclusive, however, as to theirindividuality. Here they separate out just as do the ambulacralossicles forming the mouth of the skeleton of Asteroid
On the Structure and Affinities of Palaeodiscus and Agelacrinus . 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 behaviour of the inner seriesin the mouth region was quite conclusive, however, as to theirindividuality. Here they separate out just as do the ambulacralossicles forming the mouth of the skeleton of Asteroids. This D 2 36 Mr, W. K. Spencei. On the Structure and [Mar. 2 9, separation, whicli is quite distinct, causes tlie inner series to occupya position wliich is incompatible with the view that they are theinturned edges of the outer series of the ambulacral plates. Traces of the aboral ambulacrum may be seen squeezed through onto the ventral surface. The ambulacral plates in this region as noticedby Gregory appear much narrower than those of the ventral surface*.. W^-\^~^- Text Fies, 4 and 6.—Portions of Half Pyramid of JPalceodiscus feroso. Fig. A,mteYxmlTiew,, processus lahialis, , linem eminenies pyramidales;(?., cut away portion of pyramid. Fig. 5, external view ; , concayeouter surface ; r., ridges on pyramid. In two of the inter-radii examined were portions of pyramids. It wasfound possible to reconstruct the proximal portion of one of lateral surfaces still preserved the ridges which served for theattachment of the interpyramidal muscles. They were concave, asthey are in the young Echinoid (Loven 11), and thus presentanother of the embryonic features so peculiar to Palseodiscus. Theexternal surface is deeply cut away. The median elevation forms awell-defined processus labialis. Towards the proximal end of theexterior surface there is a notch, which is continued distally in thesubstance of the pyramid. This I cannot find represented in recentUrchi
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectproceed, bookyear1904
