308 BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 



Variations. The distal avicularia are inconstant; they are sometimes atten- 

 uated (fig. 16) and even absent (fig. 18). 



The keel of the ovicell, very clear in figure 17, is much attenuated in figure 15. 



We have observed (fig. 17) an aperture closed by a calcareous operculum. 

 This phenomenon is frequent in all the Cheilostomes, but as yet zoologists have not 

 learned its significance. 



Certain zoaria are narrow at their lower extremity (fig. 18) as if they were 

 formed of segments for articulation. The base of such segments (fig. 20) bears 

 three pores, which is not the case in articulated zoaria. 



The zooecial walls (fig. 19) are very thin. The ovicell occupies a portion of 

 the cavity of the distal zooecia. As in the other species of the same genus the 

 costules are apparently not hollow. 



Affinities. This species is distinguished from all others of the genus by its 

 cylindrical zoarium, but it is very close to Metracolposa robusta of which it might 

 be a variety if we had intermediate forms. It differs from the latter in having a 

 larger number of costules, in the presence of two larger distal avicularia and in 

 the smaller zooecial breadth, and from Metracolposa grandis in its much smaller 

 lacunae and its very different micrometrical dimensions. 



Occurrence. Middle Jacksonian: Wilmington. North Carolina (common); 

 near Lenuds Ferry, South Carolina (very common). 



Cotypes.C&t. No. 64034, U.S.N.M.' 



Genus CORBULIPORA MacGillivray, 1895. 



1895. CorbuKpora MACGII.LIVKAY, Monograph Tertiary Polyzoa Victoria, Transactions Royal 

 Society Victoria, vol. 4, p. 60. 



"Zoarium erect, zooecia quadriserial, facing to the four sides, much elongated, 

 calcareous, readily separating longitudinally. Anterior part wider, raised, formed 

 by a series of vertical ribs on each side, .turning abruptly inwards and uniting to 

 form a flat plate; posterior part of zooecia narrow, smooth, and entire; thyrostome 

 with the upper lip arched, thickened; and smooth." (MacGillivray.) 



Genotype. Corbulipora omata MacGillivray, 1S95. 



Range. Eocene-Miocene. 



This genus is purely zoarial. The figures given by MacGillivray are incom- 

 plete. Two specimens that have been sent to us by Mr. Maplestone are somewhat 

 different from MacGillivray's description. The costules have lumen pores; the 

 initial slit 'separating two costules is very small. The genus has the costular struc- 

 ture of GephyTotex. 



CORBULIPORA COLLAKIS, new species. 



Plate 43, fig. 14. 



Description. The zoarium is free, quadriserial, bifurcated. The zooecia are 

 distinct, separated by a prominent thread or a furrow, and are much elongated, 

 elliptical; the frontal is convex; the costules are fine, numerous, ornamented with 

 very small lumen pores; separated by a large, initial slit, followed by two lacunae; 

 the superior costules are more or less united to form a sort of collar. The aperture 



