712 BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 



Diameter of the peristome 0.12 mm. 



Distance between the peristome ; 0.30-0.40 mm. 



Separation of the peristomes 0.52 mm. 



Variations. The zoarial variations are very great. The simple zoaria are 

 typical Discosparsa with large zoarial margins and wide zone of growth. The 

 basal lamella is striated concentrically (fig. 4), sometimes pedunculate. Some- 

 times the zoaria are superposed in the form of Domopora (figs. 7, 14) ; but more 

 often they are aggregated in a manner to form large, colonies (Multisparsa) having 

 a measurement of 2 centimeters in diameter. This zoarial proliferation occurs 

 by rejuvenescence; each zoarium is emitted from a vigorous tube of another zoarium 

 which thus serves as a pseudoancestrula. It is remarkable to note how an identical 

 phenomenon is the cause of numerous zoarial forms as varied as they are absolutely 

 unexpected. 



On the zoarial margins, the tubes are close together and are grouped in radial 

 rows and even in short fascicles (fig. 13) an arrangement, similar to that in 

 Actinopora. 



The. tubes are often closed by smooth diaphragms. According to Waters, 

 this arrangement reveals a long peristomie, of which the diaphragm is the base. 

 The aspect of our fossils is not therefore the real one. 



The ablation of the basal lamella reveals the habitual lozenge-shaped areas, 

 showing that the tubes grow one upon another from their lower dorsal (fig. 9). 



The phenomenon of rejuvenescence begins always in a central zooecium 

 (figs. 5, 6) prolonged more or less directly from the primitive zoarial 

 ancestrula. In this kind of Domopora, the peristomie, well visible (figs. 10) in the 

 median sections of the isolated zoaria, disappears completely (fig. 14). 



Finally, it is not unusual to encounter incompletely developed ovicells 

 (fig. 12). ' 



This species is especially well developed at Wilmington, North Carolina, and 

 Lenuds Ferry, South Carolina. The specimens from other localities are small 

 and doubtful. 



Affinities. The simple and free forms differ from Plagioccia discoidea in 

 which the tubes have the same diameter in that the tubes are grouped in radial 

 rows on the zoarial margins and in the greater separation of the peristomes (0.52 

 and not 0.32 mm.). 



Occurrence. Middle Jacksonian; Near Lenuds Ferry, South Carolina (com- 

 mon) ; Wilmington, North Carolina (very common). 



Upper Jacksonian (Ocala limestone) : Chipola River, east of Marianna, 

 Jackson County, Florida; Old Factory, about one and one-half miles above 

 Bainbridge, Georgia (rare). 



Cotypcs. Cat. No. 65303. U.S.N.M. 



PLAGIOECIA DIVAGANS, new species. 



Tlate 121, figs. 1-7. 



Description. The zoariiun incrusts shells, bryozoa, and algae on their flat sur- 

 faces; it is orbicular or eccentric; the zone of growth is thick but rather narrow, 



