Oil the Fe7iestellidcB — Irish Carboniferous Strata. loi 



weathered surfaces of fossiliferous limestones, and from the free 

 use of sections. 



The general character of the Feyiestellidce is as follows : — 

 The zoarium forms a delicate calcareous mesh-work, on one 

 surface of which the cellules or zooecia occur. These are 

 grouped along the main bars or colum^i^ of the mesh. The 

 columns bifurcate as the zoarium grows broader from its 

 base, and the adjacent ones either approach and join one 

 another at intervals, then separating again, or are united by 

 little cross-bars called dissepiniejits. In the former case, 

 elliptical apertures, called fenestrules, are left between the 

 sinuous bars ; in the latter case the fenestrules, elliptical or 

 rectangular, are bounded on each side by the main columns, 

 and above and below by the dissepiments. The fenestrules 

 are far larger than the zooecial apertures. 



Ulrich^ admits the genus Thainniscus into the family, and is 

 thereby forced to extend his definition to forms that possess 

 no fenestrules ; but this seems an unnecessary complication. 

 The genera included by different authors vary somewhat, but 

 amount to about fourteen, and the greatest development of 

 the family as a whole is in the Carboniferous period. For 

 our purposes, the following genera are of immediate interest, 

 and will be briefly discussed in order : — 



1. Phyllopora, King {Retepora auct.) The columns are rounded, and 

 are sinuous in the plane of the zoarium, uniting with one another 

 laterally and leaving practically circular fenestrules. The zoarium is 

 funnel-shaped when perfect, as in so many of the Fenestellids, and the 

 zooecia open on its outer surface, forming two or more rows on each 

 column. Orduz'ician to Permian. 



2. Polypora, M'Coy ^ The columns are round, and connected by 

 dissepiments. The zooecia are in 2 to 8 rows (Ulrich) on each column ; 

 M'Coy observed 3 to 5 rows. There is no keel between the rows, but 

 sometimes (Ulrich) a line of strong tubercles occurs along the column. 

 Sihtrian to Permian. 



3. Fenestralla, Prout. lAke Polypora, hvLtvfith a. ridig&or keel a. long 

 each column, on each side of which there are two rows of zooecia. Only 

 one species is known, from the Lower Carboniferous of the United States. 



^ These were unfortunately styled " interstices" by M'Coy, Young, and 



others, a name more suggestive in the intervening apertures in the mesh. 



^"Palaeozoic Bryozoa," Geological Survey of Illinois, vol. viii. (1890), 



P- 395- 



' Synopsis of the Characters of the Carboniferous Limestone Fossils of 

 Ireland (Dublin, 1844), p. 206, 



