82 THE AMERICAN MONTHLY [Mar., 



47. PERIPH^NA. 

 Pores 20-22 on radius of disk: solid girdle nearly as broad as radius of 



medullary shell decora. 



48. SETHOSTYLUS. 



Disk spiny ; 3-4 series of conical marginal spines spicatus. 



49. PHACOSTYLUS. 



Disk smooth; solid girdle with 100-120 teeth .maximus. 



50. TRIACTISCUS. 



Disk thorny ; pores 8-9 on the radius tripodiscus. 



51. HELIOSESTRUM. 



No girdle ; diam. of disk 6 times that of medullary shell solarium. 



No girdle ; diam. of disk 4 times that of medullary shell contiguum. 



Girdle ; diam. of disk 3 times that of medullary shell craspedotuTu. 



52. HELIODISCUS. 



No girdle ; 60-80 marginal spines helianthus. 



Girdle radially striped ; 16-20 teeth on margin huml)oldti. 



Girdle not radially striped ; 10-20 teeth on margin umbonatus. 



53. ASTROPHACUS. 



Surface smooth ; girdle radially striped; 18-24 marginal spines cingillum. 



.54. LITHOCYCLIA. 



Phacoid shell surrounded by 7-11 chambered rings ; pores 9 on radius of 



phacoid shell ocellus. 



Phacoid shell surrounded by 5-8 chambered rings ; pores 7 on radius of 



phacoid shell monococcus. 



55. STYLOCYCLIA. 



Phacoid shell surrouded by 5-8 chambered rings dimidiata. 



Phacoid shell surrounded by 4 chambered rings excavata. 



56. AMPHICYCLIA. 



Phacoid shell spongy, twice as broad as outer medullary shell... pachydiscus. 



.57. STAUROCYCLIA. 



Phacoid shell surrounded by 3-4 spongy rings ; the 4 crossed spines with 



serrated edges serrata. 



58. ASTROCYCLIA. 



Phacoid shell surrounded by 4-8 rings stella. 



Phacoid shell surrounded by 3 rings heterocycla. 



59. COCCOCYCLIA. 



Numerous marginal spines arranged in several circles heliantha. 



