TERMINOLOGY. 27 



The areola, or scrobicule (PI. VI, fig. 1, d), is the round, oval, or eUiptical, smooth, 

 excavated space which surrounds the base of the boss. This space is sometimes narrow, 

 and its margin elevated into a ridge, that completely encircles it, when the scrobicule 

 or areolar circle is said to be complete ; if the upper and under sides of the circle are 

 wanting, it is incomplete, and the areolas are then said to be confluent. 



The row of granules encircling the areolas is called the areolar or scrobicular 

 circle. 



The miliary zone is the space comprised between two ranges of primary tubercles, 

 it is in general covered with a close-set granulation ; Avhen destitute of miliary 

 granules, it is said to be naked ; the ambulacral and inter-ambulacral miliary zones 

 occupy these different regions of the shell. 



When the granules are microscopic, and closely clustered together within certain 

 narrow, circumscribed bands, or fascioles, intersecting the general tubercular surface, and 

 occupying fixed positions on the test, they are ceiWed fasciolar. 



Fascioles are only found in two families, in one genus of the Echinocoridee, and in 

 almost all the genera of the Spatanc/idce. The form, width, structure, and position of these 

 bands must be carefully noted, as they afford important characters, both positive and 

 negative, for the diagnosis of the genera. 



When a fascicle surrounds the circumference of all the petaloidal portions of the am- 

 bulacral areas, it 1% peripetalous ; if it encircles the single ambulacrum alone, it is internal ; 

 when it extends along the flanks, it is lateral ; if it passes in whole, or in part, along 

 the circumference, it is marginal ; and when it surrounds the base of the single inter- 

 ambulacrura, it is suhanal. Sometimes there is only one., sometimes there are two or 

 three, of these fascioles in different genera. 



Each family has a special arrangement of the tubercles, granules, and fascioles ; a 

 detailed account of these will be found in the general outline of the structural characters 

 prefixed to the description of each natural group. 



The sutural impressions are the lines along which the plates are united together ; some- 

 times they are mere lines ; or the impressions amount to excavations out of the borders 

 of the plates, and give rise to cavities therein, as in the genus Temnopleurus. 



The angular or sutural pores are small impressions, situated in some genera at the 

 angles of the plates, upon the median line of the miliary zones. 



The external Appendages of the Test. 



The spines, or radioles, as they were called by Plott, Lang, and other old authors, are the 

 calcareous appendages that are moveably articulated to the tubercles of the test ; they 

 present numerous modifications as to size, form, and sculpture, which are all intimately 



