622 PALAEONTOLOGY OF ILLINOIS. 



PTILOPORA ACUTA Ulrich. 



PI. LXV. Fig. 4-4a. 



Zoarium a large pinnate frond, the largest fragment seen 

 having a width of 3 cm. from the central shaft to the lateral 

 margin. 



Reverse. Mid rib strong, from 1.0 to 1.5 mm. in width, ap- 

 pearing like a cylindrical stem lying on the surface of the frond, 

 and marked with distinct longitudinal striae. Branches rather 

 slender, generally a little flexuous, springing from the shaft, 

 eight or nine in 1 cm., at angles varying in different examples 

 from 22 to 42. As in the specimen figured the angle may be 

 quite different on the two sides of the shaft. Still, 30 seems 

 to be about the normal. The average width of the branches is 

 about 0.3 mm., but in extreme cases it may vary from 0.2 to 

 0.5 mm. Measuring at right angles to their direction, from 

 thirteen to fifteen branches occur in 1 cm. Dissepiments slightly 

 depressed, on a level with the branches, or somewhat elevated 

 above them, usually a little wider than the branches, but vary- 

 ing in width from 0.3 to 0.7 mm. Surface of branches and dis- 

 sepiments finely striated. Fenestrules long-oval, about 1.3 by 0.5 

 mm., with six in 1 cm. 



Obverse face not well shown by any of the material before me, 

 but, so far as can be determined, shows, that the midrib has a 

 strong, rounded and striated keel, two ranges of zocecia, with 

 large apertures separated by a space about equal to their 

 diameter, and that there are fourteen apertures in 5 mm. 



The larger frond, more rounded midrib, and more acute angle 

 at which the branches and midrib meet, distinguish this species 

 from the associated P. valida. The Warsaw and St. Louis P. 

 prouti has much shorter fenestrules, and closer and less acutely 

 arranged branches. 



Position and locality: The types are from the Keokuk group 

 at Bentonsport and Keokuk, Iowa, but what seems to be the 

 same species is represented in the Illinois State Museum by a 

 specimen from the Burlington limestone, at Burlington, Iowa. 



