764 BULLETIN 106, UNITED STATES NATIONAL, MUSEUM. 



they border the zoarial margins and are almost opposite on each side of the 

 median crest. The tubes are visible convex, separated by a slight salient thread ; 

 the peristome is thin, round or rectangular. The dorsal is hardly convex; it is 

 formed by a thick layer of dactylethrae closed by a thin calcareous lamella. The 

 dactylethrae appear on the frontal at the bifurcations. 



Diameter of the tubes 0.16-0.20 (maximum 0.30 mm.). 



Distance between the fascicles 0.80-1.00 mm. 

 Measurements. ,, 7 . ,,, ,, , , ,, . , 



Width of the fascicles.- 0.24 mm. 



Width of the branches 1.20 mm. 



Variations. This species is quite fragile; the fascicles are very often broken 

 and the dactylethrae have lost their covering pellicle. Specimens like figure 6 

 are much more frequent than the good specimens (figs. 2, 3). 



In longitudinal section the walls of the dactylethrae (fig. 9) often appear 

 hollow ; the intercellular tissue is therefore incomplete. The nature of dactylethrae 

 is easy to see on our figures, but their function is unknown to us. We have not had 

 the fortune to find the growing extremity of a branch near the zoarial base. 



The tangential section of the tubes is analogous to that of all other Cyclosto- 

 mata. 



Affinities. The species differs from Erkosonea admota in the great distance 

 between the fascicles and in the absence of radicular lamellae on the dorsal. 



Occurrence. Lower Jacksonian (Moodys marl) : Jackson, Mississippi (com- 

 mon). 



Cotypes. Cat. No. 65364, U.S.N.M. 



ERKOSONEA ADMOTA, new species. 



Plate 139, figs. 14-22 ; plate 133, fig. 8. 



Description. The branches of the zoarium are large, linear, bifurcated with 

 oval transverse section. The fascicles are salient, close together, oblique, divergent, 

 formed of five zooecia; they are arranged alternately on each side of the median 

 crest and do not spread beyond the zoarial margins. The tubes are visible, very 

 little convex ; the peristome is thin and rectangular. On the dorsal the dactylethrae 

 have very salient margins; there are wide radicular lamellae. 



Diameter of the tubes 0.20 mm. 



Distance between the fascicles 0.50-O.GO mm. 



Width of the fascicles 0.20 mm. 



Width of the branches 1.5 mm. 



The dorsal is hardly convex ; it frequently bears very wide radicular lamellae. 

 These are formed of two rows of dactylethrae which bend around at right angles. 

 In Idmonea grattator there are the habitual branchings of the tubes which form the 

 radicells. The remarkable facility with which the Cyclostomata modify their 

 tubes for adaptation proves that the zoarium is not only an animal colony but a 

 veritable real individuality. The vital unity is assured in the interior of the 

 zooecia by the mesenchymatous tissue. 



Measurements. 



