DEVELOl'MEN'l' OK i'EUONOl'OliA .'{(il 



teristics of these buds have already been desei'ibed. Siiiiihir doubt exists 

 as to the exact derivation of the buds marked a, h, c, and d in the figures. 

 Tlie most natural supposition is tliat a and h came from bud ]. Tlie 

 buds z and z' turn abruptly backward and inward toward tlie antero- 

 posterior axis of the zoarium, in exactly the same fuaiiiier as similarly 

 placed buds in the recent Bryozoa. 



Development oe Pkronopora 



the primary buds 



It will not be necessary to take up in detail the development of this 

 genus, since in all essential particulars it is identical with Frasopora. 

 ^riie protoecium has not been seen. Oidy two specimens showing Ww, 

 initial region of this genus have so far been ()l)tained. The best of these 

 is shown in figures 13-15. It is a nearly entire basal ex})ansi()n and 

 shows not only the initial buds and ancestrula, but the manner of origin 

 of the median lamina that characterizes the genus. Three buds arise 

 from the ancestrula, as in Prasopora, and are bounded by the character- 

 istic thickened, structureless wall (.t-.t'). There appears to be an addi- 

 tional bud between the buds corresponding to e and /, as is also the case 

 in some specimens of Prasopora. The beautifully symmetrical arrange- 

 ment of the zooecia is shown in the diagrammatic drawing, figure 13. 



THE MEDIAN LAMINA 



The initial region of Peronopora for some distance from the ances- 

 trula is identical with Prasopora. At a distance of 2 or 3 millimeters 

 from the ancestrula, however, the zooecia in Pcroihopom begin to diverge 

 more and more from an imaginary line, and a little farther out are sepa- 

 rated into two juxtaposed regions by a definite median plate. This plate, 

 the median lamina, is encountei'cd at about the same distance from the 

 ancestrula whether the section cuts Ncrtically or horizontally through the 

 initial I'cgion — that is, the zone in which the lamina makes its (ii-st ap- 

 ])earance arches over the initial region in the antero-])()sterioi' vertical 

 ])lane. A young colony a few millimeters in diameter may therefoi'c be 

 said to be substantially a minute Prasopora. The lamina itself seems to 

 represent the suppressed axial region of an otherwise flabellate zoarium. 

 In other words, instead of having a well developed axial region, consist- 

 ing of the thin walled immature portions of many longitudinally diivcted 

 long zooecial tubes, Peronopora has reduced the axial region to a double 

 lamina, consisting of the juxtaposed proximal ends of o])positely directed, 

 short zooecial tubes standinij- at riiiht an<>-les to the lamina. It is clear 



