Measurements. 



772 BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 



they are alternated on each side of the median line and adjacent to each other on 

 this line. The tubes are visible, flat, separated by a scarcely salient thread. The 

 basal lamella is striated transversally ; the zoarial margins are thick and distinct. 



Diameter of the tubes 0.06 mm. 



Distance between the fascicles 0.12-0.20 mm. 



Width of the fascicles 0.10 mm. 



Width of the zoarium 0.8 mm. 



A-ffkiitiesIn. the number of the tubes to the fascicles this species approaches 

 Idmonea arcuata; it differs from it in the separation of the fascicles which is less 

 than 0.20 mm. and in the rectilinear form of its zoarium. 



In the zoarial margins of the basal lamellae the species resembles Idmonea 

 petri D'Archiac, 1846, but differs from it in the ensemble of its much smaller 

 micrometric dimensions and in its claviform zoarium. 



Occurrence. Midwayan (Clayton limestone) : One mile west of Fort Gaines. 

 Georgia (rare). 



Cotypes.Cni. No. 65246, U.S.N.M. 



IDMONEA MAGNA, nev species. 



P'late 137. figs. 1-18. 



Description. The zoarium is large, dichotomously branched, sometimes retic- 

 ulated, enlarged at the bifurcations, triangular, higher than wide in transverse 

 section. The fascicles are quite salient, oblique, alternated on each side of the 

 median crest and quite distant from it ; they bear four to eight zooecia. The tubes 

 are visible, convex, almost all equal. The basal lamella is striated transversally, 

 flat, somewhat convex or a little concave; the tubes are often visible; the zoarial 

 margins are very narrow and 'somewhat thickened. 



Diameter of the tubes 0.34 mm. 



Distance between the fascicles 0.70 mm. (0.40-0.80 mm.). 

 Measurements. ,, , . , 



Width of fascicles 0.24-0.30 mm. 



Zoarial width 1.40-2.00 mm. 



Variations. The. number of the tubes to the fascicle is more often four or 

 five (figs. 2, 3) ; at the bifurcations it is larger (eight) (fig. 9). The basal lamella 

 is convex (fig. 5) or concave (fig. 10) ; the two forms are visible on the same frag- 

 ment (fig. 8). The fascicles of the reticulated branches are closer (fig. 4). At 

 the bifurcations the fascicles are transverse (fig. 2), if the branches rest in the- 

 same plane; when the branches are twisted and are opposite, the fascicles remain 

 distinct and alternate (fig. 9). .. On well-preserved specimens the tubes are visible 

 on the basal lamella (fig. 6). The incomplete zooecia in the process of formation 

 are quite numerous (fig. 10) and give a very crowded appearance. 



In transverse section (fig. 18) the zoarium is triangular and the zooecia are 

 polygonal; but seen from the interior, by abrasion of the basal lamella, they are 

 round (fig. 15). The longitudinal section (fig. 11) is normal; the tubes grow 



