Gainings — Development of some Paleozoic Bryozoa. 71 



in number, are defined (figs. 72, 73, a-g). In Polypora the dis- 

 tal margin of the zocecia is elevated, so that by careful inspection 

 of the initial region of a young colony, the initial (central) 

 zooecium can be readily identified. In transverse sections, also, 

 of the base of adult colonies the primary zocecia can be easily 

 determined (fig. 77). In the section shown herewith, the pro- 

 tcecium, because of its lower position, and more intimate con- 

 tact with the substratum, had been broken open from beneath 

 through its thin basal plate, and infilled with the iron-stained 

 material of the matrix, thus making it a very conspicuous object 

 in the transparent section. The primary zocecia are arranged 

 very symmetrically about the protcecium, — one in front, two 

 to the right, and two to the left. There can be no doubt that 

 the individual in front and the anterior pair of lateral indi- 

 viduals represent the median and lateral buds. Whether the 

 posterior pair of lateral individuals represents another set of lat- 

 eral primary buds or was derived from the anterior lateral buds 

 cannot at present be definitely settled. Certain silicified speci- 

 mens of Polypora^ showing the initial region at the base of the 

 zoarium, make it practically certain that the zocecia in question 

 were derived from the anterior lateral buds. In Retepora 

 phaznieea, which has exactly the same arrangement of zocecia 

 about the protcecium as Polypora, these posterior buds are 

 .undoubtedly derived from the anterior laterals.* The buds 

 marked II in fig. 77 are manifestly not derived from the 

 protcecium. 



If the posterior pair of laterally placed buds is not primary, 

 i. e. 9 derived from the protcecium, the stage represented in fig. 

 77 is to be regarded as metanepiastic, since the branching and 

 ascent of the colony from the surface of support commence 

 with the formation of the next circle of buds. Fig. 73 shows 

 an individual just entering the neanastic stage. The meta- and 

 para-stages are therefore greatly abbreviated in Polypora. 



Comparison icith Fenestella. 

 It requires no very minute inspection of the nepiastic stages 

 of Fenestella and Polypora (cf. figs. 77 and 57 ; 76 and 46 ; 73 

 and 47 ; 74 and 52) to reveal a profound difference in the develop- 

 ment of the two genera. The median bud of Polypora arises in 

 front of the protcecium, instead of on top as in Fenestella. The 

 protcecium of Polypora is surrounded by subsequent zocecia and 

 therefore comes to occupy a central position. In Fenestella the 

 protcecium remains one of a more or less complete circle of 

 zocecia, and is in no sense central in position. The "zooecial 

 apertures face upward and toward the axis of the zoarium in 



*The budding order has been determined by the writer, in Retepora 

 phcenicea, from St. Vincent's Gulf, S. Australia. Specimens showing the 

 protcecium alone, and the protcecium and three primary buds, are now in the 

 Yale University Museum, and will be described in a later paper. 



