256 PAL/EONTOLOGY 



body, a slight projection at one point forming a rudi- 

 mentary stem, while almost opposite it is the slightly- 

 raised rim of a circular opening, presumably the mouth. 

 About one-third the distance from the mouth to the stem 

 is a third projection a low pyramid of five triangular 

 plates, the anal pyramid. Between this and the mouth, 

 but nearer the latter, is a very small round opening, 

 interpreted as a hydropore, that is an opening by which 

 water is taken into the svstem of water- vessels. 



FlG. 72. ECHINOSPH^RA AURANTIUM, GYLLENHALL, ORDOVICIAN, 



SWEDEN. 

 (Natural size. Original.) (Only a few of the plates are indicated. ) 



A, Anal pyramid ; //, hydropore ; M, mouth. 



The spherical theca is made up of a very large number 

 of polygonal (mostly hexagonal) thecal plates, which when 

 closely examined show a pattern somewhat resembling 

 that on the infrabasals of Marsupites, though due, not as 

 in that to external ornament, but to the internal structure 

 of the plates. The stereom of each plate is thrown into 

 folds perpendicular to the adjacent suture, so that the 

 whole surface appears divided up into a series of rhombs, 

 each belonging half to one plate and half to another, 

 the folds in each rhomb forming a parallel series. The 



