220 H. T. JACKSON — STUDIES OF PALiEECHINOIDEA. 



These two plates, 1 and 2, in the several areas of Archxocidaris by additions 

 form the two adambulacral columns of plates (compare with Melonites, 

 plate 2, figure 2). In the next row of interambulacrals there are 3 plates, 

 plate 3 being the initial j^late of the first median column of interambu- 

 lacral plates. In the next row above initial plate 3 we find in the 5 areas 

 the introduction of a fourth plate, 4, which is the beginning of the fourth 

 column of plates. It is seen in areas A and C that the fourth column 

 originates in its theoretically correct position, with two columns on the 

 left and one on the right. In areas E, G and /, however, the fourth 

 column is represented as beginning on the left of the center, with one 

 column of plates on the left and two on the right. 



Continuing dorsalh^, we find that 4 columns continue to be formed, and 

 no more are known in this species. In the first row added above the 

 introduction of the fourth column in this diagram (plate 8, figure 45) 

 a circular dotted line bisects the plates. All below or on the ventral side 

 of this dotted line is reconstruction, but that portion which lies above 

 the line represents the condition as actually observed in the specimen of 

 Archseocidaris from which })late 8, figure 43, was drawn.^ The plates c, d, 

 e and /above the dotted or resorption line in plate 8, figure 45, in the sev- 

 eral areas correspond in form and position exactly with those of plate 8, 

 figure 44. It is to be observed that in order to make these plates come 

 in their proper order it was mechanically necessary to have column 4 

 right-handed in its development in areas A and C and left-handed in 

 areas E, G and I. It is also to be noted that, arranging them as I have, 

 the columns are introduced precisely as in Melonites (plate 2, figure 2, 

 and figure l,})age 164), and carrying out the relative proportion in size, 

 as seen in other genera, the number of plates introduced in the recon- 

 struction is just sufficient to build the required number of columns and 

 to meet around what was the border of the peristome at an early stage 

 in growth. 



STRUCTURE OF LEPIDOCIDARIS SQUAMOSUS. 



jNIessrs Meek and Worthen (31) founded the genus Lepidociddris for the 

 single species L. squamosus, M. and W. The type of the species and genus 

 (plate 7, figure 41), which was in the Wachsmuth collection, is now in the 

 collections of the Museum of Comparative Zoology (catalogue number 

 3026). The species is from the lower Burlington limestone of Burling- 

 ton, Iowa. Besides the type, there are several fragmentary specimens of 

 the species in the Cambridge museum. This species differs in man}^ fea- 

 tures from Archxocidaris^ so that Meek and Worthen's genus Lepidocidaris 

 should certainly stand, and I think Professor Keyes is mistaken, when in 



* I*jxcept for restoration of area /, which is reconstructed, as shown in phite 8, figure 44. 



