PROF. E. W. CLA.YPOLE ON HELICOPOPvl. 31 



" Genus Fenestella, Lonsdale. 



" Polyzoary a calcareous reticulate expansion, either flat, conical, 

 or cup-shaped, formed of slender bifurcating branches (interstices), 

 poriferous on one face, connected by non-poriferous bars (disse- 

 piments) forming an open network. Cells immersed in the inter- 

 stices, and arranged in two longitudinal rows divided by a central 

 keel, on which are often prominences. Cell-mouth small, circular, 

 and prominent when preserved." 



Accepting this definition of Fenestella, it is highly probable that, 

 when the day shall come for a thorough revision of the North- 

 American Fenestellids, some of the 60 or more species at present 

 standing under this name will be excluded or synonymized. 



The closer and closer definition of the genus Fenestella has had 

 the natural and necessary effect of excluding species after species, 

 which have been placed in new genera or subgenera from time to time 

 established. In this way the forms with a strong, stony, central shaft 

 and spiral polyzoary have been set aside as Archimedes. Polypora 

 includes those forms with more than two rows of true pores. Hall 

 has grouped a number of species in which the expanded frond is 

 stiffened by stony marginal arms on one or both edges as Lyropora. 

 Ptilopora includes all forms possessing a stony midrib and foliate 

 fenestrate fronds on each side. Thus those species which possess 

 none of these characters remain in and compose the present genus 

 Fenestella. 



The following details in regard to the groups which have been 

 successively separated from Fenestella will place their relations in a 

 clearer light. 



1839. Fenestella, Lonsdale and Miller. As defined by Lonsdale, 

 this genus included the forms having three or more rows of 

 cells, whether on the rays or .the bars, such as Polypora, 

 Retepora, &c. (Murch. Geol. of Russia, App. A, p. 629). 



1842. Archimedes, Lesueur and D. D. Owen. No description was 

 published with the figure on which this genus is based, and 

 which appeared in the ' American Journal of Science and Art ' 

 for July 1842. Prof. Hall says, in the ' Palaeontology of 

 Iowa,' p. 651, " The Bryozoans designated as above by Lesueur 

 do not differ in their essential structure from Fenestella ; their 

 mode of growth, however, is quite distinct, the flabelliform 

 expansion acquiring a solid central axis, around which it 

 revolves in an ascending spiral form, spreading equally in all 

 directions." 



1844. Ptilopora, Scouler and M'Coy. " Flabelliform or infun- 

 dibuliform, attached by roots, from which a strong midrib 

 arises, giving origin on each side to thin equidistant interstices, 

 connected by regular dissepiments ; external face of the 

 interstices carinate, and bearing two rows of pores." (Synopsis 

 of Carb. Foss.) 



1844. Polypora, M'Coy. " Corallum forming a delicate, reticulated, 



