PSEUDODIADEMA. 109 



tubercles ; or they have four, or even six rows of nearly equal-sized primary tubercles at 

 the equator. 



The poriferous zones are narrow and straight ; the pores in one section are unigeminal 

 throughout ; but in another section they are bigeminal in the upper part of the zones. 



The apical disc is seldom preserved ; it is, in general, small ; and the anterior pair of 

 genital plates are larger than the posterior pair. 



The mouth opening is large, the peristome is deeply notched, and the oral lobes are 

 not very unequal sized. 



The spines rarely attain the length of the diameter of the shell ; they are in general 

 much shorter, and are cylindrical or needle-shaped, and have a prominent, milled ring near 

 the articulating head ; the rim of the acetabulum is crenulated, and the socket perforated ; 

 the surface of the stem is sculptured with delicate longitudinal lines. 



The Pseudodiadema are all extinct, and are found in the Liassic, Oolitic, Cretaceous, 

 and Tertiary rocks. 



Pseudodiadema differs from Diadema in having solid spines, with a smooth 

 surface, the sculpture, in most cases, consisting of microscopic, longitudinal lines ; whilst 

 the spines of Diadema are tubular, and have oblique annulations of scaly fringes on their 

 surface. It differs from Cyphosoma, which is a Cretaceous genus, in having the tubercles 

 always perforated, those of Cyphosoma being imperforate. 



It differs from Hemipedina in having a small apical disc, and tubercles with crenulated 

 bosses, those of Hemipedina being smooth; and from Pedina in having the pores 

 unigeminal or bigeminal, those of Pedina being arranged in triple, oblique pairs. 



The Pseudodiademas may be divided into two sections, from the different manner the 

 pores are arranged in the zones. In one group the pairs of pores are not so numerous, 

 and they are disposed in a single file throughout ; in another group the pores are more 

 numerous, and crowded together in the upper part of the zones. Professor M'Coy* has 

 proposed the genus Diplopodia for this group. Caterus paribus, the crowding together 

 of a greater number of pores in a zone is, at most, a sectional, and can never form a 

 stable generic character, inasmuch as it is subject to great variation in the diplopodous 

 species themselves, and is, moreover, often only an adult development. 



Annals and Magazine of Natural History,' 2d series, vol. ii, p. 412. 



