454 PALEONTOLOGY OF ILLINOIS. 



PETALOTRYPA COMPRESSA Ulrich. 



PI. XLVI, fig. 4-4f. 



Zoarium consisting of compressed branches, two or more cm. 

 in height, one cm. or less in width, and form one to four mm. 

 in thickness. Surface smooth, with scarcely appreciable clusters 

 of cell apertures a very little larger than the average, and with 

 somewhat wider interspaces. Zooecial tubes almost direct from 

 the medial plane, the prostrate portion very short. Walls of 

 moderate thickness throughout. Zooecia sub-polygonal, at times 

 quite regularly hexagonal, about eight in two mm. The num- 

 ber of mesopores varies in different examples; disposed to con- 

 gregate in the clusters. Diaphragms abundant in the zooecia, 

 situated at rather unequal distances apart; in the mesopores 

 more closely set. Dark spots simulating acanthopores, or per- 

 haps really of that nature, found at some of the angles between 

 the zooecia. Between two adjoining zooecia there is a dark line 

 which is interrupted by tissue of the same color as the wall 

 substance. I am not prepared to give the exact interpretation 

 of this feature. 



This species neither superficially nor internally can be con- 

 fused with any other species known to me from Devonian rocks. 

 Very frequently specimens are overgrown by a form of Aulopora 

 often causing the subsequent growth to be very irregular. 



Position and locality: Hamilton group; Davenport, Iowa; 

 Rock Island, 111. 



PETALOTRYPA DELICATA Ulrich. 



PL XLVI, fig. 5-5a. 



Zoarium a slightly undulated bilaminar frond, about two cm. 

 in height, the same in width, and 0.7 mm. in thickness. But 

 two examples of this species have been examined. Surface 

 smooth; cell apertures angular, subrhomdoidal or hexagonal, 

 arranged in diagonally intersecting series, about nine in the 

 space of two mm. measuring diagonally. Zooecial tubes very 

 short, approaching the surface in a steady curve. An occasional 

 diaphragm present at the beginning of the cortical region. 

 Acanthopores absent, though at some of the angles of the zooecia 

 are dark spots which may be of that nature. 



