266 



BULLETIN 11, UNITED STATES NATIONAL MUSEUM. 



distinctly marked, off from each other by a black divisional line. 

 This dark line is sometimes broken or traversed by areas of the same 

 light-colored tissue making up the wall itself. 



Genoty])e. — Petalotrypa compressa Ulrich. Middle Devonian of 

 Illinois and Iowa. 



PETALOTRYPA FOLIUM, new species. 

 Text fig. 157. 



Zoarium a delicate, bifoliate expansion about 4 cm. in both height 

 and width and not exceeding a millimeter in thickness. Surface 

 smooth, with clusters of cell apertures, appreciably larger than the 

 average, at intervals of about 3.5 mm. Zooecial tubes regularly 

 polygonal, opening directly at the surface, seven in 2 mm. Walls of 

 the zooecia thin and without acanthopores; mesopores either wanting 

 entirely or present only in the maculae. 



Fig. 157.— Petalotkypa folium, a, sueface of the type-specimen, X9, sho-\ving a macula m the 



LOWER HALF OF FIGURE; b, TANGENTIAL SECTION, X20; C, A FEW ZOCECLA. OF THE SAME, X40, EXHIBITING 

 THE DARK DIVISIONAL LINE; d, VERTICAL SECTION, X20, SHOWING BIFOLIATE GROWTH AND SIMPLICITY 

 OF INTERNAL STRUCTURE. KEGEL LIMESTONE (D2), HABBINEM, ESTHONIA. 



In thin sections the zooecial tubes are seen to arise rather abruptly 

 from each side of the mesial lamina. As the surface is approached 

 their walls thicken shghtly and the dark line separating adjoining 

 zooecia is developed. Vertical sections likewise indicate the absence 

 of diaphragms. 



The dark divisional line is especially well shown in tangential sec- 

 tions, which exhibit also the simphcity of zooecial structure as a 

 whole. In figure 157 h, a few mesopores or interspaces are seen to 

 be limited to the macula of larger or more irregularly shaped ceUs, 

 while the zooecia proper are smaller and assume a quite hexagonal 

 outline. 



The present species agrees in all respects with the Devonian forms 

 upon which the genus was based, indeed, it is quite close to Petalotrypa 



