452 PALEONTOLOGY OF ILLINOIS. 



True mesopores are also wanting, but in several species, notably 

 M. crassimuralis and M. subquadrata, the zocecial walls are 

 partially separated near the surface by tabulated interspaces. 



MONOTRYPELLA CKASSIMURALIS Ulrich. 

 PL XXXVIII, fig. 2, 2f. 



Zoarium dendroid, dividing dichotomously or otherwise at in- 

 tervals from ten to eighteen mm.; branches with low rounded 

 monticules, whose centers are about two mm. apart. Zocecial 

 tubes after passing through the axial region with a steady 

 curve, open at the surface with direct circular apertures, 0.1 

 mm. in diameter, arranged in regular curved series, about eight 

 in two mm. Interspaces thick, flattened centrally, then sloping 

 down to the zooecial cavities; thickest on the monticules, where 

 a few illy defined mesopores are generall distinguishable. Tan- 

 gential sections vary considerably in the appearance of the in- 

 terspaces, sometimes showing open spaces of diverse form and 

 size between the ring-like walls of the zocecia, (see PI. XXXVIII, 

 fig. 2c); at other times the interspaces seem to be filled with a 

 light colored calcareous deposit. In transverse sections the 

 tubes in the axial portion of a branch are very thin walled and 

 regularly rhomboidal or pentagonal. Diaphragms wanting in 

 the axial, very few in the mature region; numerous and thick 

 in the interspaces. 



This species belongs to the same section of the genus as M. 

 quadrata Kominger, and M. subquadrata Ulrich, though differ- 

 ing very obviously from them. All three agree in having the 

 tubes in the axial region regularly rhomboidal. Another pecu- 

 liar feature is the habit of changing the direction of the rhombs 

 at intervals of about one mm. This peculiarity is readily no- 

 ticed in rough vertical fractures, which exhibit concentric, alter- 

 nately smooth and rough or toothed spaces, each about one 

 mm. wide. In thin vertical sections each change is marked by 

 the origin of a number of rapidly enlarging young tubes. 



The most striking feature of the species is the extreme thick- 

 ness of the interzooecial spaces. This character alone readily 

 distinguishes the species from its nearest allies. 



Position and locality: Cincinnati group, Wilmington, 111. 



