ECHINOIDEA. 231 



masticatory apparatus. The Regular Echinoids comprise the 

 families of the Cidaridce, Hemicidaridce, Diademadce, Echin- 

 idce, and Saleniadce. 



In the Cidaridce, the test is spheroidal and more or less 

 ilattened at the oral and anal poles. The ambulacral areas 

 (fig. 124), are very narrow, often flexuous, and never pro- 

 vided with large tubercles. The interambulacral areas are 

 wide, and carry large perforated tubercles, surrounded by an 

 areola, and supporting the primary 

 spines. The spines are of two 

 sizes, the primary ones usually 

 more or less cylindrical, clavate, 

 or fusiform, and generally longi- 

 tudinally ridged or tuberculate 

 (fig. 122). Of the genera of this 

 family, Cidaris itself (fig. 124) is 

 the most important, ranging from 

 the Trias to the present day. 

 Rhabdocidaris is Jurassic and Cre- f p -. 12 ? u ? d ? surfa f c ' of the + tes * 



of Cidaris vcsiculosa, of the natural 



taceous ; Diplocidaris is Jurassic ; size, chalk. (After Wright.) 

 and the Porocidaris of the Secon- 

 dary and Tertiary periods has now been detected by Sir 

 Wyville Thomson in a living condition. 



In the Hemicidaridce (fig. 121) the test is spheroidal, 

 more or less depressed, and with a close general resemblance 

 to that of the Cidaridce. The ambulacral areas, however, 

 though narrow and mostly undulated, are wider than in the 

 Cidaridce, and are provided with comparatively large tuber- 

 cles, which may be developed inferiorly only (Hemicidaris), 

 or which may extend along the entire length of the area 

 (Acrocidaris). The interambulacral areas are wide, and carry 

 very large perforated tubercles with crenulated bosses. The 

 spines are usually long, cylindrical, and tapering. The type- 

 genus of this family is Hemicidaris (fig. 121), which ranges 

 from the Upper Trias to the Lower Cretaceous inclusive. In 

 Acrocidaris, of the Jurassic period, the ambulacral areas are 

 comparatively wide. 



In the family of the Diademadce sometimes made to 

 include that of the Hemicidaridce the test is circular or 



