756 SYSTEMATIC PALEONTOLOGY 



(second cycle), and these split 0.5 mm. from the center into four (third 

 cycle) ; about 1.5 mm. from the center each of the four costae divides to 

 form eight (fourth cycle), and about 2.5 mm. from the center in the type 

 each of the eight divides, producing sixteen costae (fifth cycle) on the outer 

 rim. The bifurcations of each cycle are at nearly equal distances from the 

 center. The costas up to the cycle of forty-eight are relatively thick and 

 coarsely nodular; those of the last cycle are thin, finely denticulate, and 

 form a band about f mm. wide, bordering the outer margin ; they appear 

 not to project beyond the edges of the septa. The intercostal loculi are 

 very narrow and are occupied by twelve or thirteen synapticulae separated 

 by perforations, most of which are slightly elongated radially; the syn- 

 apticulaa and perforations are arranged in concentric rows. 



The septa are very thin and are arranged in six groups, one group in 

 each of the interspaces between the primary septa. Total number of septa 

 ninety-six. The secondaries extend to the columella; the tertiaries fuse 

 against the secondaries near the columella ; the two outer quaternaries of 

 the group fuse against the tertiaries nearer the center than do the two 

 inner ones; in each of the two subgroups formed about the tertiaries the 

 two outer quinaries fuse against the quaternaries nearer the center than 

 do the two inner ones. The primary septa are slightly higher than the 

 members of the higher cycles which appear to be of about equal height. 

 On the sides of the corallum the septa distinctly alternate in prominence. 

 Margins of the septa finely denticulate, the number of denticulations 

 being about ten to 1 mm. Sides of septa with strias and rows of syn- 

 apticulffi and tubercles radiating from near the base of the columella. 



Columella elliptical, spongy, trabecular, some of the trabeculae termi- 

 nating in more or less scattered, irregularly distributed, small papillae; 

 length of cross-section between one-fifth and one-sixth the diameter ; width 

 about one-tenth the diameter. 



This species differs from the other species of Micrabacia as follows : 1 

 M. kilgardi differs in size, form, and ornamentation of the base. The 

 corallum is smaller, the sides straighter and more inclined, and the septal 



1 The species mentioned in this paragraph, with the exception of M. ameri- 

 cana and M. coronula, are described by the writer in Prof. Paper U. S. Geol. 

 Survey, No. 98J, now in press. 



