io2 *rhe Irish Naturalist. 



4. Fcncstella, Lonsdale. The columns are round, and are united by 

 dissepiments, slighter than themselves, as in Polypora. There is a keel, 

 as in Fenestralia ; but there is only one row of zooecia on each side of it. 

 Silurian to Permian ; most abundant in Carboniferous. 



4a. Archimedes, Lesueur. IaIlq Fenestella, but wound spirally 

 about a central axis, with the zooecia on the internal or upper face. 

 Carboniferous, 



4b. Ptriopora (Ptylopora), M'Coy^ (Scouler MS.). Like Fen- 

 cstella, but columns diverging pinnately on each side of a central 

 and thicker axis. Carboniferous. 



5. Semlcoscfnium, Prout. (= Carinopora^ Nicholson). Two rows of 

 zooecia, as in Fenesiella, but the dissepiments are thicker and shorter, and 

 the keel on each column is greatly developed. This striking feature is 

 often thickened near its crest or in its central portion, and its form can 

 be well studied in sections.- Silurian to Devonian. 



6. Unitrypa, Hall. Like semicoscinium, but the crests of the prominent 

 keels send out cross-bars which connect them. Sometimes there are 

 two bars to each fenestrule, and sometimes there is one to each zocecium. 

 Uppermost Silurian to Devonian. 



7. Isotrypa, Hall. Like Semicosciniiuii, but the keels are thin at first 

 and then expand, the long plate-like summits that are thus produced 

 being connected by bars at regular intervals ; these bars correspond to the 

 dissepiments beneath them, and form a sort of outer mesh work. Silurian 

 to DeiJ0)iia7i. 



8. Hem itry pa, Phillips.^ In this genus the correspondence of structure 

 between the outer mesh and the inner fenestrated zoarium is carried 

 farther than in Isotrypa^ and a delicate network, which may be styled the 

 legmen,^ covers the face of the zoarium, and is supported by pillars rising 

 from keels like those of an ordinary Fenesiella. Each row of pillars, in 

 fact, bears a rod running parallel to the column of the zoarium which 

 lies beneath it, and these rods give off bars, producing a network between 

 each pair of rods. The circular apertures of this network correspond to 

 the zooecia underlying them. Silurian to Carbonifei-ous. 



In the above synopsis the genera are arranged with an eye 

 to their culmination in the exquisite details of Hemitrypa, 

 which was at one time nearly consigned to oblivion by 

 European palaeontologists, its tegmen being somewhat negli- 

 gently regarded as a parasite/ The range of this genus makes 



1 Op. cit. p. 200. 



-See, for instance, Nicholson and Lydekker, "Manual of Palaeontology," 

 vol. i., p. 625, fig. 469 G. 



3 " Palaeozoic Fossils of Cornwall, Devon, and West Somerset." 

 (Ordnance Geological Survey, 1841), p. 27. 



^ G. A. J. Cole, " On Hemitrypa hibernica," Proc. Royal Dublin 80c., vol. 

 viii. (1893), p. 137. 



^Ibid.^i^. 133-5. ....... 



