REGULAR EUECHINOIDS. 



33 1 



two minor groups of the Diadematidce and the Echinidce, both of 

 which appear in the Secondary period, and are represented at the 

 present day. 



In the Diadematidce, the pairs of ambulacral pores (fig 253) form 

 a single row on each side of the ambulacral areas, except near the 

 apex and the peristome, where more than one row of pore-pairs 

 may be developed. The test is generally circular or pentagonal, 



Fig. 253. — Portion of the test of Pscudodiadema Fittonii, enlarged four times, a, Ambulacral 

 area; /, Interambulacral area. Lower Greensand (Cretaceous). (After Wright.) 



more or less depressed and flat below. The ambulacral areas are 

 wide, and carry two rows of large primary tubercles, equal in size to 

 the two or more rows of tubercles upon the interambulacra. The 

 tubercles are sometimes perforated, sometimes imperforate, and 

 they may or may not be crenulated. The spines are cylindrical and 

 slender, and usually of considerable length. In the living Diadema 



the spines are 



long, 



tubular, and covered with imbricated scales 



Fig. 254. — Hemicidaris crenularis, showing tubercles, the larger of which are perforated, 

 and are surrounded by an areola. Jurassic. 



arranged in oblique rings. The Diadematidce, are very extensively 

 represented in the Secondary rocks, the earliest forms appearing in 

 the Upper Trias ; the Tertiary forms are less numerous, and a 

 limited number of types referable to the group still exist. 



Of the genera of this family, Hemicidaris (fig. 254) has a spheroidal, 

 more or less depressed test, with a general resemblance to that of the 



