4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I40 



septa" in the original description of the type species, A. septosa 

 (Ulrich, 1879, p. 125). (For further description of offset acantho- 

 pores see pp. 19, 21.) 



Of the approximately 18 Middle Ordovician species of Am- 

 plexopora, described and undescribed, in the U. S. National Museum 

 collections, offset acanthopores causing inflection of zooecial walls 

 are limited to a single undescribed species from the Cannon lime- 

 stone near the top of the Trenton, Pulaski, Giles County, Tenn. 



Of the approximately 14 Upper Ordovician species of Am- 

 plexopora, described and undescribed, in the U. S. National Museum 

 collections, offset acanthopores causing inflection of zooecial walls 

 were found in all but one species, A. variahile (Ulrich), 1890, from 

 the top of the Richmond group, Osgood, Ind, 



Most of the Middle and Upper Ordovician species of Aniplexopora 

 referred to above have been described or differentiated on one to 

 several sectioned specimens and are therefore necessarily typologi- 

 cal in taxonomic approach. Despite the theoretical limitations of 

 typologically defined species, enough specimens are available to indi- 

 cate that offset acanthopores are characteristic of Amplexopora in the 

 Upper Ordovician rocks of the Ohio and Mississippi River valleys. 



Within the present concept of Amplexopora at least three inferred 

 lineages appear concurrently in rocks of Middle and Upper Ordovician 

 age in North America. One lineage is characterized morphologically 

 by A. conferta (Coryell) of Black River age, A. cylindracea Ulrich 

 and Bassler of Trenton age, and A. Columbiana Ulrich and Bassler, 

 of Maysville age, and seems to be geographically centered in the 

 Central Basin region of Tennessee. The Upper Ordovician species in 

 this first lineage develop offset acanthopores but not as strongly as 

 those occurring in a second lineage characterized by the type species, 

 A. septosa (Ulrich). This second lineage seems to be centered in the 

 Ohio, Indiana, and northern Kentucky region, but this impression is 

 no doubt partly due to the predominance of collections from that area 

 in the Museum's Upper Ordovician material. 



A third lineage is suggested by species such as A. winchelli Ulrich 

 from the Decorah shale in the Upper Mississippi Valley and A. 

 cingidata Ulrich and A. rohusta Ulrich in formations of the Maysville 

 group in the Ohio, Indiana, and northern Kentucky region. In this 

 third lineage, the Upper Ordovician species develop offset acantho- 

 pores, which are so small in diameter that they rarely produce an 

 extreme petaloid or "pseudo-septal" appearance in tangential sections. 



