658 PALAEONTOLOGY OF ILLINOIS. 



In young examples the arrangement appears to be rather more 

 regular than in the larger ones. Measuring longitudinally, or 

 transversely, five or six in 1 mm. Interspaces rather thin, 

 rounded, not appreciably different in young and old examples, 

 carrying small tubercles about equal in number to the zooecia 

 apertures. 



Of internal characters those furnished by vertical sections are 

 the most interesting. (See PI. LXXI, fig. 3c.) 



Large examples of this species especially remind one very 

 much of such TREPOSTOMATA as Batostomella, but thin sections 

 prove beyond question that it is a true species of Rhombopora, 

 with close relations to R. wortheni and R. simulatrix. From 

 the former it is distinguished by the ramose habit of growth, 

 and much less regular arrangement of the zooecia apertures. 

 The latter differs in having more slender branches, thicker and 

 granulose interspaces, and more regularly arranged zooecia 

 apertures. 



Position and locality: St. Louis group. Monroe county, 111. 



RHOMBOPORA TABULATA Ulrich. 



PI. LXX, fig. 2-2<3. 



Zoarium a cylindrical stem from 1 to 1.5 mm. in diameter, 

 branching dichotomously at intervals of 10 mm. more or less. 

 Zooecia apertures ovate, overaging 0.18 mm. in length, from 

 one-half to two-thirds as wide, arranged in irregular series, with 

 the transverse and diagonal lines less frequently dominant than 

 the longitudinal. On an average five apertures occur in '2 mm. 

 transversely, and from twelve to fourteen in 5 mm. longitudin- 

 ally. Measuring diagonally, seven is the usual number in 2 mm. 

 Interspaces carinate, as wide or wider than the zooecia aper- 

 tures, carrying at most of the angles of junction a moderately 

 large tubercle. Sloping areas varying considerably in form, being 

 sometimes hexagonal, and at other times pentagonal, lozenge- 

 shaped, or irregularly quadrate. 



Thin sections, aside from showing that the species is rather 

 closely related to R. persimilis, are interesting because of the 

 comparative abundance of true diaphragms in the axial or 

 primitive portion of the zooecia tubes. These and other char- 

 acters of a vertical section are well shown in fig. 2c. 



