266 PALEONTOLOGY 



others are flat : in the middle of this side is a large 

 opening, the periproct. The mouth, however, is still in 

 the centre of the oral surface, and the very small apical 

 disc is equal central at the aboral apex. 



The smoothness of the surface enables the sutures of 

 the plates to be traced easily. The interambs are broad 

 and composed of large, nearly oblong plates, with from 

 one to twelve or more larger flat tubercles and numerous, 

 much smaller tubercles (granules). The ambs are narrow ; 

 their plates are much smaller and more irregular, includ- 

 ing simple primaries, simple demi-plates, and some com- 

 pound plates ; each has one or sometimes two larger flat 

 tubercles, and a number of granules. 



The peristome is small ; the lantern and perignathic 

 girdle are in a vestigial condition. The apical disc is 

 not very different from that of Hemicidaris, except that 

 the madreporite is larger than the other genital plates 

 and extends into the space vacated by the periproct. 



Micraster cor-anguinum (Fig. 77) is another sea- 

 urchin from the Upper Chalk. Viewed from above or 

 below its outline is heart-shaped, the anterior amb being 

 deeply depressed so as to notch the outline. The other 

 ambs are also sunk in a similar way for some distance 

 from the apical disc, but not far enough to affect the 

 outline. The posterior interamb is raised into an obtuse 

 ridge, the keel or carina, which runs almost horizontally 

 back from the apical disc to the hind margin which 

 drops perpendicularly. Sometimes the carina projects 

 slightly above this vertical end, forming a rostrum. The 

 anus lies near the top of this vertical face. 



