Cumings — Development of some Paleozoic Bryozoa. 63 



In rare instances the initially nabellate zoarium curves in the 

 reverse sense and by fusion of the meeting edges forms an 

 infundibular colony which has literally been turned inside out. 

 Such zoarial modifications have given rise to the maximum of 

 confusion in the placing of species and genera of fenestrate 

 Bryozoa, because in the adult colony lacking the base (as is 

 almost invariably the case), the fundamental plan of budding 

 is entirely obscured. 



The carinse originate during the metanepiastic stage. 



jRaranepiastic /Stage. — Successive tiers of zocecia are added 

 above the initial circle until a cylindrical stem is formed which 

 constitutes the stalklike base of the adult cone. The parane- 

 piastic stage may be greatly abbreviated where the cone 

 expands almost from the first circle of zocecia, or it may be 

 prolonged to five or six tiers of zocecia (fig. 55). If such a 

 cylindrical stalk is conceived of as slit longitudinally down the 

 side diametrically opposite the protcecium and unrolled and flat- 

 tened out, it will be seen that the protcecium lies at the base of 

 a vertical series of zocecia from which lateral branches are 

 given off on either side. In other words, the budding follows 

 Davenport's law that the median bud continues the ancestral 

 row. It is not certain, however, that any importance is to be 

 attached to this law. 



Near the close of the paranepiastic stage the axis of the 

 zoarium thickens preparatory to the expansion of the cone. 

 This is well shown in figs. 55 and 56 (longitudinal sections) 

 and figs. 58 and 62 (transverse sections) of specimens from the 

 Hamilton of Thedford, Ontario. The thickened axis (ax) is 

 composed of an outer wall continuous with the proper wall of 

 the carinse, and an inner dense deposit of punctate scieren- 

 chyma. The section shown in fig. 60 cuts just at the top of 

 this thickened axis. 



Description of Figures 55-60. 



Figures 55-60. — Longitudinal and transverse sections of bases of Fen- 

 estella from the Hamilton formation of Thedford, Ontario. 



55. Section of a Fenestella base cutting exactly in the plane of the axis 

 and of the zocecial apertures to the right (1 to 10). The initial zocecium (pro- 

 tcecium) is at o ; the thickening of the axis (ax) commences at b' ; the apex 

 of the cone of expansion of the colony is at p ; the vesicular tissue (c') 

 above p is of secondary origin, forming during the mature and senile life of 

 the colony. Fig. 56 shows the thickening of the axis, and oblique direc- 

 tion of the zocecia, but does not cut through the zocecial apertures. 57. 

 Transverse section through the initial region of another individual (about in 

 the plane of 1, fig. 55) ; a-e, primary carinae ; 1 to 10, primary row of 

 zocecia. 58. Transverse section through the thickened portion of the axis 

 (about in the plane of ax, fig. 55) of another individual; a-e, primary 

 carinse (see also fig. 62). 59. Transverse section in the plane of c', fig. 

 55, showing nine carinas. 60. Transverse section in the plane of p, fig. 

 55, showing, in the lower portion of the figure, a carina just in process of 

 branching. 



All figures x 13. 



