GENERA. AjN^D aKOFPS OF THE ECHINOIDEA. 71 



Ambulacra narrow, with two vertical rows of perforate, plain, 

 primary tubercles ; the poriferous zones with diplopodous pairs, 

 which are numerous, close vertically, distant horizontally, and 

 regular. Plates numerous, low, broad, composed above the 

 ambitus, as elsewhere, of three small plates ; the adoral and 

 aboral are low broad demi-plates and the middle plate is a primary 

 which carries the tubercle and occupies the median suture. Pairs 

 of pores of a compound plate really in low broad triplets ; only 

 every alternate compound plate has the adoral pair of pores 

 nearest to the ambulacral median line. 



Interradia with many low broad plates, with many vertical rows 

 of perforate primaries slightly larger than those of the ambulacra, 

 ceasing at the ambitus, and largest there ; and two rows extend 

 vertically to the apex. 



Peristome moderate, star-shaped, with broad projecting ambu- 

 lacral ends, having a well-marked branchial incision and a tag on 

 eitlier side, the interradial margins being more pointed and 

 narrower than those of the ambulacra. Peristomial membrane 

 with imbricating plates. 



Jaws and teeth " diadematoid." Spines short, slender, and stri- 

 ated above the ambitus, slender and blunt club-shaped actiually. 



Recent. Philippines, Fiji Islands ; 100-600 fathoms. 



Greuus Plistophtma, Feron Sf GautJiier, 1881, Ech. Foss. de 

 V Alger, fasc. 8, p. 176, pi. xx. 



Test moderate, circular at the tumid ambitus, very depressed, 

 nearly three times as broad as high. 



Apical system large and subpentagonal. 



Ambulacra narrow, straight, with two vertical rows of imper- 

 forate plain primary tubercles. Poriferous zones with the pairs 

 of pores biserial, actinally and dorsally, and in simple series at the 

 ambitus. 



Interradia broad at the ambitus, the median area distinct and 

 bare abactinally ; four or five vertical rows of primary tubercles 

 in each (or more); tubercles resembling those of the ambulacra, 

 tliose next to the median suture the largest and passing up to 

 the apex, rows oblique ; plates numerous, low, broad, and oblique, 

 especially at the ambitus. 



Perjstome large, pentagonal ; branchial incisions small. 



Fossil. Cretaceous : Europe and IS". Africa. 



