ECHINODERMATA. 15 



zone, and the inter-ambulacral miliary zone, when they occupy these different regions of 

 the shelh 



When the granules are microscopic, and are closely clustered together to form certain 

 narrow, circumscribed bands or fascioles which intersect the general tubercular surface, 

 and occupy fixed positions on the test, the microscopic granules composing these zones 

 are called /(25C2o/ar. 



Fascioles are only found in two famihes, in one genus of the EchinocovT/dcB, and 

 in almost all the genera of the Spatangidce. The form, width, structure, and position of 

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

 negative, which serve in the diagnosis of the genera. 



When the fasciole surrounds the circumference of all the petaloidal portions of the 

 ambulacral areas, it is csHiieA. peripetaloiis ; when it encircles the single ambulacrum alone, 

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

 in part along the circumference, it is marginal: and when it surrounds the base of the 

 single inter-ambulacrum, it is sub-anal .• 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, which 

 will require a detailed notice 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. 

 Sometimes they are mere fines (PI. I, fig. 4 3; PL II, figs. 13, 2 c); sometimes the im- 

 pressions 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 Eadiolii as they were called by Plott, Langius, and other old authors, 

 are the calcareous appendages which are moveably articulated with the tubercles of the 

 test. They present numerous modifications as to size, form, and sculpture, which are all 

 intimately connected with specific characters. Some are short, elongated, flattened, cylin- 

 drical, fusiform, or subulate ; others are compressed, spatuliform, or triangular ; others, on 

 the contrary, are expanded, pyriform, or claviform. The surface of the spines is smooth, or 

 striated with fine or coarse longitudinal lines ; some have verticellate processes at regular 

 intervals ; others have asperities, prickles, or granules, disposed with more or less regu- 

 larity over the surface; the different parts of the spine have received the following 

 names : 



The articular cavity, or acetahidum, is the socket by which the spine articulates with 



