NiCKLEs AND liAHSLKit.] DIAGNOSES OF GENERA. 39 



above and connectod by thin o])liqiio .sul)imbrieating plates (scalte), 

 which may ])o as numerous as the zooecia, or only two to each fenestrulc. 

 (xenotype: Fenestella {Ilemitnjpa) lata Hall. Silurian, Devonian. 

 Sixteen species. 



Hemitrypa Phillips. 



Zoarium as in Fenestella, but has a reticulated superstructure whose 

 meshes correspond in number and position with the zocecial apertures; 

 the structure rests on pillai's which rise at regular intervals from the 

 moderate median keel of the branches. 



(xenotype: Hemitrypa oculata Phillips. Range, Silurian-Carbon- 

 iferous. Sixteen species. 



Helicopora Clay pole. 



Zoarium spiral, the inner edge thickened and nonporiferous, but 

 without forming a solid axis; otherwise as in Fenestella. 



Genotype: Helicopora latispiralis Claypole. Silurian, Devonian. 

 Two species. 



Archimedes Owen (Archimedipora D'Orbigny). 



Zoarium a spirally wound fenestrated expansion supported by a solid 

 calcareous, central axis; otherwise as in Fenestella. 



Genotype: Fenestella {Archimedes) wortheni Hall. Mississippian. 

 Sixteen species. 



PoLYPORA McCoy (Protoretepora De Koninck, Polyporella Simp- 

 son, Flabelliporella Simpson). 



Zoarium as in Fenestella but has from two to eight rows of zooecia on 

 a branch, and lacks the median keel, though this is sometimes repre- 

 sented by a row of strong nodes or tubercles. 



Genotype: Polyjwra dendroides McCoy. Range, Silurian-Carbon- 

 iferous. Eighty-two described and other new species. 



Lyropora Hall (Lyroporella, Lyroporina, Lyroporidra Simpson). 



Zoarium flabellate, the fenestrated portion spread between the arms 

 of a nonporiferous U or V shaped calcareous support, which is free or 

 pedunculate at the base; zooecia in from two to five ranges; no median 

 keel. 



Genotype: Fenestella {Lyropora) suhquadraiis Hall. Mississippian. 

 Seven species. 



Fenestralia Prout. 



Zoarium as in Fenestella, but with four rows of apertures, two on 

 each side of the prominent median keel. 



Genotype: Fenestralia sanctl-ludovlci Prout. Mississippian. Two 

 species. 



