ECHINOIDEA. 



369 



thus continuously perforated from pole to pole, but they are " in- 

 terrupted," only their upper portions being regularly poriferous. 



Fig. 239. — Echinoconus cotizcus {=Galerites albogalerus). The first figure shows the under 

 surface with the mouth and anus ; the middle figure is a side-view ; and the right-hand figure 

 shows the upper surface, with the ambulacral areas converging to the apical disc. White Chalk. 



In such cases (fig. 240) the ambulacral zones are widened out 

 superiorly, and form a kind of rosette upon the upper surface of the 



,V 



It, 



A: 



Fig. 240. — Scutella subrotunda, showing petaloid ambulacra. Miocene. 



test, when they are said to be " circumscript " (ambulacra circum- 

 scripta), or "petaloid." 



The most important external structures of the corona are the 

 tubercles and spines. The tubercles are rounded elevations upon 



241. — Hemicidaris crenularis, showing tubercles, the larger of which are perforated, 

 and are surrounded by an areola. Jurassic. 



which the spines are carried (fig. 241). They vary much in their 

 dimensions, and receive special names, according to their size or 

 position on the test. Ordinarily the tubercle consists of a rounded 

 vol. 1. 2 A 



