Cumings — Development of some Paleozoic Bryozoa. 69 



branches in double rows in precisely the same way as in the 

 latter genus. The presence of scalse is the only distinguishing 

 character. In Unitrypa as in Fenestella the zocecia are always 

 on the exterior surface of the cone. 



Homology of Parts in Unitrypa and Fenestella. — The 

 carinse are undoubtedly homologous in the two genera. They 

 do not, however, reach the bottom of the cup in Unitrypa, or 



69 



Figure 69.— Plan and elevation (semi-diagrammatic) of young Unitrypa of 

 the stage shown in figs. 63-68. a-e, primary carinae : a'-e', primary 

 branches; pi-ps, transverse plates (scalae). The zocecia are numbered to 

 correspond to fig. 61. Three rows are present below the margin of the cup 

 and two above. The probable orientation of the exserted polypide is shown 

 by the arrow* at 4. The basal plate is indented opposite each zocecium as in 

 the young of Fenestella, x 26. 



at least if they did their inner portion was too delicate to be 

 preserved (figs. 64, 67, 68). The cup is in appearance unlike 

 anything to be found at a corresponding stage in Fenestella. 

 Nevertheless there are. strong reasons for considering it as 

 homologous, in the main, with the basal plate of the latter. 

 The rim of the cup may, however, represent one or more of 

 the earlier scalse fused with the margin of the basal plate. The 

 scalloping of the rim suggests this latter interpretation. The 

 cup might very well overspread the entire outer surface of the 

 zoarium were it not for the necessity of adequate circulation of 

 water over the zocecia. This condition is approximated in some 

 types (Isotrypa, Semicoscinium). 



