196 POLYCYPHUS. 



Genus— POLYCYPHUS, Agassiz. 1 846. 



This genus consists of small urchins, with a sub-globular or hemispherical form; the 

 sides and upper surface of the test are covered with numerous, small, equal-sized, imper- 

 forate tubercles, which form numerous vertical and horizontal rows ; at the base of both 

 areas the tubercles are disproportionately large, when compared with those on the upper 

 surface. 



The poriferous zones are wide and depressed, and the pores are arranged in triple 

 oblique pairs ; near the peristome the pores are more numerous, and crowded. 



The mouth opening is large ; the peristome is pentagonal, the notches are shallow, 

 with wide ambulacral, and short inter-ambulacral lobes. 



The apical disc forms a narrow ring, and the genital and ocular plates are small. 



The wide inter-ambulacral areas are each divided into two lobes by a slight median 

 depression, so that the surface of the test in this genus is divided into fifteen nearly equal- 

 sized lobes. 



Polycyphus very much resembles Magnolia in the general fades of the species, in the 

 size and ornamentation of the test, in its division by depressions into lobes, in the number of 

 small imperforate tubercles which crowd its surface, and the ring-like character of the apical 

 disc ; but Magnotia is distinguished from Polycyphus by the narrowness of the ambulacral 

 areas, and especially by having the pores strictly unigeminal throughout the narrow sunken, 

 poriferous zones. 



Polycyphus resembles Stomechinus in having the pores in triple oblique pairs ; but is 

 distinguished by the number and general uniform character of the tubercles, arranged in 

 vertical and horizontal rows, and by having those at the base of the areas disproportion- 

 ately large when compared with the small tubercles on the upper surface. In Stomechinus 

 there is always a primary row of tubercles, and secondary rows ; but in Polycyphus they 

 are in general uniformly of the same size. The peristome in Stomechinus is always very 

 deeply incised by large notches, whilst in Polycyphus the division of the peristome is but 

 feebly marked. 



