SUB-GENUS MONOTRYPA. 



they exhibit their most highly characteristic shape, being 

 strictly rhombic or square, and being arranged in regularly 



f'g- 36. — A, A fragment of Monliculipora quadra ta, Rominger, from the Cincinnati Group of 

 Ohio, of the natural size ; B, A tangential section of the same, passing just below the 

 calices, enlarged eighteen times ; C. Part of the preceding section, enlarged fifty times, show- 

 ing the structure of the walls of the corallites ; D, Part of the central area of a transverse 

 section, showing the peculiar rhombic form of the tubes in this region, enlarged eighteen 

 times ; E, Part of a vertical section, enlarged eighteen times. 



decussating oblique lines. Thin as their walls are here, there 

 is usually a more or less conspicuous nodal enlargement at 

 each of the angles of junction of the corallites. The cavities 

 of the tubes are filled with transparent calcite, and each has 

 its rhomboidal area very distinctly and regularly divided into 

 four equal triangles by a cruciform divisional line. These 

 divisional lines in the interior of the tubes are perfectly regu- 

 lar in their arrangement, and are quite uniform in their direc- 

 tion in each specimen : they therefore give rise to a second, 

 fainter, double series of diagonal lines, which intersect the 



