4 Field Museum of Natural History — Geology, Vol. IV. 



the lateral fasciole (If) which branches from the peripetalous fasciole 

 and passes beneath the periproct. 



Tubercles: (PL II figs. 8,9.) Wart-like protuberances which occur on 

 the plates of most echinoids. They vary in size from quite large to 

 microscopic granulations. Tubercles consist of two parts, the boss 

 and the mamelon. The base or boss may be either plain or crenulated. 

 The mamelon is a small rounded protuberance above the boss to which 

 in life, a spine is attached. A plain or sunken space surrounding the 

 boss generally marked by a ring of granules is called the areole or 

 scrobicule. 



Periproct (pt) : The upper or posterior opening in the shell, im- 

 properly called the anus, but which contains the latter. The periproct 

 is closed by a membrane to which the anal plates are attached. These 

 plates may be few and symmetrical, or numerous and irregular, and 

 arranged concentrically about the anal opening. The position of 

 the periproct is within the apical system in "regular echinoids" but 

 in "irregular echinoids" it is situated somewhere in the posterior 

 interambulacral area. It may be either on the dorsal or ventral 

 surface. The position of the periproct is of great systematic impor- 

 tance. 



Peristome (ps) : The lower opening of the shell ; it is situated on 

 the ventral surface, either at the center or on the median line between 

 the center and the anterior margin. In form it may be decagonal, 

 pentagonal, oval, bilabiate, etc. It contains the buccal membrane 

 in the center of which the mouth is situated. 



Labrum: (1) The posterior lip of the peristome. 



Proximal: Nearest to the apex. 



Distal: Farthest from the apex. 



Lateral: Pertaining to the side. 



