232 



ANNULOIDA. 



pentagonal, more or less depressed and flat below. The 

 ambulacral areas are wide, and carry two rows of large 

 primary tubercles (fig. 125), 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 



Fig. 125. Portion of the test of Pseiulodiadema Fittonii, enlarged four times, a, Ambul- 

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



and slender, and usually of considerable length. In the 

 living Diadema and in Astropyga the spines are long, tubu- 

 lar, and covered with imbricated scales arranged in oblique 

 rings. As the type of the family we may take Pseudodia- 

 dema, which ranges from the Liassic to the Tertiary inclusive, 

 and in which the spines are solid and microscopically striated, 

 while the tubercles are perforated (fig. 125). In the Cretace- 

 ous Cyphosoma the tubercles are solid. Hemipedina, again, of 

 the Oolites, is very like Pseudodiadema, but the tubercles are 



Fig. 126. Goniopygus iruijor, viewed from above and sideways, of the natural size. 

 Cretaceous. 



not crenulated. We may also place here the Cretaceous 

 genus Goniopygus (fig. 126), which presents us with a type 

 intermediate between the Diademadce and Saleniadce. It ap- 

 proaches the latter family more particularly in the great size 



