﻿20 SMITHSONIAN .MISCELLANEOUS COLLECTIONS [VOL. 47 



but distinct, one or two to each zooecium. Mesopores restricted 

 almost entirely to the maculae. Curved or oblique diaphragms very 

 frequent, from one-half to nearly one tube-diameter apart. 



This species is related to 0. wetherbyi (Ulrich), but has smaller 

 and less frequent acanthopores. 0. lamellosa (Ulrich) — see plate 

 vi, IO , — has decidedly larger zocecia besides numerous and larger 

 acanthopores. 



Occurrence. — Not uncommon in the Pierce division of the Stones 

 River formation at Murfreesboro. Tennessee. 



Cat. No. 43.174. U. S. N. M. 



Genus Prasopora Nicholson and Etheridge 



PRASOPORA PATERA new species 1 



(Plate VI, n-14) 



Zoarium of subcircular, almost flattened disks, 30 or 40 mm. in 

 width and usually 3 or 4 mm. thick. Under surface more or less 

 concave, often nearly flat, covered with a concentrically wrinkled 

 epitheca. Near the center of this face there is usually a cicatrix or 

 the body itself (commonly a valve of Dalmanella) upon which 

 growth commenced. Surface smooth, the maculae not raised into 

 monticules but easily distinguished by the large size of some of the 

 zocecia contained in them. 



Zocecial apertures generally appearing quite angular, the walls 

 b^ing thin and the mesopores small, relatively few, and, on the whole, 

 quite inconspicuous as external features. Of the intermacular zocecia 

 an average of 7 occurs in 2 mm. The largest in the maculae attain 

 a diameter nearly twice as great. 



Internal structure much as in P. simulatrix Ulrich, but not iden- 

 tical. The walls are always thinner in P. patera and its zocecia 

 usually more angular, while the mesopores, as a rule, are fewer in 

 number. 



Though closely related to P. simulatrix — a fact that was not sus- 

 pected until we prepared thin sections — the discoid or saucer-shaped 

 form so persistently maintained by the hundreds of specimens before 

 us is so strikingly different from the nearly equally constant hemi- 

 spheric or conical zoarium characterizing that species that it seems 



1 Although the purpose of these papers is to deal only with matters pertain- 

 ing to the elucidation and variations of generic groups, we have found it 

 desirahle to use this opportunity for the introduction of descriptions of a 

 ies which are characteristic fossils of certain Ordovician horizons in 

 the Central Basin of Tennessee. Figures of the external characters of these 

 species are given in the Columbia Tennessee folio, recently issued by the 

 Geological Survey. 



