218 R. T. JACKSOy STUDIES OF PAL^ECHINOIDEA. 



seem to bear any relation to the 4 columns in the corona. They impinge 

 upon the corona in a quite irregular manner, and in the most perfect area, 

 A, are seen to be 5 in number on the margin. Comparing these interam- 

 bulacral plates of the peristome with those of a recent specimen of Cldaris 

 tribuloides. Lam., from Panama, in the collections of the Museum of Com- 

 parative Zoology (catalogue number 404, plate 8, figure 48), it is seen that 

 the form is strikingly similar. Orally there is a single column of these 

 interambulacral plates in Cldaris ; but on the outer border, as pointed out 

 by Loven (27) some accessory plates are seen, as shown in the figure, and 

 as also shown in Cldaris jmplUata, plate 9, figure 55, after Loven (27). 



The ambulacral plates of the peristome in Archasocidarls ivortheni differ 

 from the same plates of the corona in being drawn out in the longer axis, 

 and they are somewhat irregular in sha})e, the plates of the corona being 

 very uniform in size and shape. These plates of the peristome interlock 

 apparently somewhat loosely on their apposed median border and later- 

 ally are drawn out in somewhat acuminate points. They have in all 

 plates observed two pores, which are situated in one horizontal plane. No 

 ambulacral plates were seen nearer the mouth than those represented. 

 Such plates if existent might show the pores arranged in a dorso-ventral 

 rather, than a horizontal position in the plates. Comparing the ambu- 

 lacral plates of the peristome of Arcluxocldarls with those of Cldaris 

 (plate 8, figure 48), we find that in Cldaris they are more regular in form, 

 relatively much larger both vertically and horizontally, and the pores 

 are arranged vertically, overlying one another, instead of horizontally, as 

 in ArcJueocl darts; also in Cldaris these plates imbricate strongl 3^ adorally. 



In another species of Archxocldarls, from Peoria county, Illinois, in the 

 Museum of Comparative Zoology (catalogue number 3029), the spines 

 and plates of jiortions of the skeleton are very clearly preserved. On the 

 plates of the corona the large primary spines are attached directly to tlie 

 large central tubercles of the plates. Small secondary spines are abun- 

 dantl}'' preserved, attached to the secondar}^ tubercles situated on the 

 borders of the plates, as in Archxocldarls ivortheni (plate 8, figure 46). A 

 portion of the peristome is also preserved, showing imbricating interam- 

 bulacral plates and elongate plates with two pores in the ambulacral 

 areas. These plates of the peristome are largely hidden by a profusion 

 of small spines which thickly cover the area. 



The peristome and ventral border of the corona have never been de- 

 scribed before in Archxocidaris, except in so far as Professor Hall's original 

 figure of this same specimen shows it ; also the plates of the peristome, I 

 think, have not been figured in any Paleozoic genus, although they are de- 

 scribed by Sir Wyville Thomson (39) in Echinocystltes ; therefore on ac- 

 count of its importance a restoration of this area is given in plate 8, figure 



