American Palceozoic Bryozoa. 



255 



Longitudinal sections (PI. XL, fig. 5c) show that the tubes in the 

 axial region have very thin walls, and are traversed by remote horizon- 

 tal diaphragms, from two to three times the diameter of a tube dis- 

 tant from each other. As they approach the surface, they bend out- 

 ward rather abruptly, their walls are much thickened, and the dia- 

 phragms become much more numerous. The tube-wall in the per- 

 ipheral region is divided into four longitudinal portions, by three 

 distinct dark lines. The two inner portions represent the original 

 walls of two adjoining tubes, and are composed of a fibrous structure, 

 the fibres being directed obliquely upward to meet along the dark 

 central line. The two outer zones, which are of a darker color than 

 the inner layers, represent the secondary deposits within the original 

 polygonal walls of the tubes. The diaphragms in the outer portion 

 of the tubes are usually nearly horizontal. All of my sections ? 

 however, show a few very peculiar diaphragms. In the section they 

 are represented by two curved plates which spring from the opposite 

 walls of a tube, nearly meeting, either in the center, or nearer one side 

 of the tube-cavity, when they proceed as nearly parallel lines down- 

 ward to the next straight diaphragm. Their shape was undoubtedly 

 that of a funnel, of which the position of the lower tubular portion, with 

 regard to the expanded mouth, was somewhat erratic. 



In transverse sections the tubes in the axial region have very thin 

 walls, and are strictly polygonal. 



The species above described I regard as the type of the genus, Am- 

 plexopora, proposed b}' me in the last number of the Journal, p. 154. 

 At the present time I am unable to give the exact limits of the genus, 

 as I have not yet fully determined where the boundary line between 

 Stenopora, Lonsdale, and Amplexopora, is most properly drawn. If 

 the periodic thickening of the tube-walls is regarded as a necessary 

 feature of species of the former, and I believe that it should be, then 

 the limits of the latter genus might be extended so as to include a 

 number of Devonian and Lower Carboniferous species (e.g. M. monili- 

 formis, and 31. barrandi, both Nicholson). At any rate the Cincin- 

 nati Group contains at least five other distinct species, which in their 

 general characters precisely resemble A. cingidata. Of these but two 

 have been described, one by the author, under the name of Atactopora 

 septosa, and the other b}^ Nicholson, original^, under the name of 

 Chaetetes discoideus, but latterly (Genus Monticulipora, p. 193) he re- 

 fers the species to his section Monotrypa, to which, however, I do not 

 find it to be more than remotely related. The fourth species (A. ro- 



