24G R. T. JACKSON — STUDIES OF PAL.EECHINOIDEA. 



42. Worthen, St. John and Miller: Geological Survey of Illinois, vol. vii, 18S3. 



43. Joseph Wright : Description ofa new species of Pa/c'«ec/it?z us. Journ. Royal Geol. 



Soc, Ireland, vol. i, part 1, 1865. 



44. Karl A. Zittel: Handbuch der Palseontologie. 



Explanation of Plates. 



In the accompanying figures the radial orientation of specimens, when expressed, 

 is designated by letters placed on the interanibulacral and ambulacral areas A, B, 

 C, etcetera, up to J, for the 10 areas, as in plate 4, figure 19. The interambulacruni 

 selected as A is a matter of indiiference, for at present I think there is no means 

 of orienting Paheechini, as in modern regular Echini, by means of the madreporic 

 body. The sequence adopted is that as viewed looking down on the corona from 

 the dorsal side and revolving like the hands of a watch, as in i)late 5, figure 20. 

 When viewed from the oral side, as in plate 2, figure 2, the same orientation is pre- 

 served, but is of necessity in a reversed order. 



The following letters are used to designate plates or series of plates in the figures : 



P and N^= unusual pentagonal plates, seen in a few interambuiacral areas. 



Terminal pentagonal plates of interambuiacral columns as introduced are num- 

 bered from 1 upward. 



-0"= heptagonal plates, seen in interambuiacral areas, usually adjacent to terminal 

 pentagons. 



«, b are the two primary ambulacral columns of amlmlacral plates. 



a^, y are the two derivative ambulacral columns seen in Oligoporus (plate 6, 

 figure 25) and Melonites (plate 2, figure 4). 



All figures except plate 1, figure 1, were drawn by Mr J. H. Emerton. The ex- 

 ceptional figure was drawn by Mr ]M. Westergren. The photographs from which 

 plate 4 was made were taken by Mr Charles H. Currier, of Boston. The source from 

 wliich all specimens or figures was derived is given in the description of plates. 



In the explanations one or two page references are given to indicate where the 

 principal description of the figure is. Many of the figures, however, are discussed 

 at several other places in the text. 



