NO. 5 ORDOVICIAN BRYOZOA BOARDMAN 3 



then is assumed to be approximately proportional to the ontogenetic 

 growth stage or stages attained by the individual asexual zooids at 

 their death. In the Trepostomata, a progressive decrease in the width 

 of the exozone and number of zooecial diaphragms is generally ob- 

 servable from bottom to top of essentially complete zoaria of appre- 

 ciable vertical extent such as ramose, frondescent, or bifoliate growth 

 habits. The ontogenetic stage attained by the oldest zooids at the base 

 of a colony then provides a measure of the astogenetic stage of the 

 colony as a whole, and the progressive decrease in exozone width and 

 number of diaphragms up through the colony to the growing tips indi- 

 cate subsequent addition of progressively younger zooids. 



A significant change in microstructure of zooecia as individuals 

 within a colony grew distally is not common in exozones of the 

 Trepostomata, so an estimate of ontogenetic stage is not generally 

 possible in tangential sections that cut zooecia and acanthopores trans- 

 versely. In A. mirahile, however, zooecial walls change from an in- 

 tegrate appearance with sharply defined zooecial boundaries to broadly 

 amalgamate with obscure boundaries. Also, wall laminae change from 

 a V-shaped to broadly U-shaped configuration in most zooecia, and 

 acanthopores start with very small diameters and become progressively 

 larger until they dominate cross-sectional shapes of mesopores and 

 some zooecia (pi. 4). The variation is unusually large and a reason- 

 ably large number of specimens is necessary to demonstrate inter- 

 mediate forms and to correlate variation with stage of development 

 of individual zooecia. 



Stratigrapkic occurrence of offset acanthopores in Amplexopora. — 

 The species restudied and included in Amplexopora in this paper 

 occur in the Middle and Upper Ordovician series. All these species 

 have acanthopores that extend through the exozones and are con- 

 centrated in zooecial corners, very similar to conservative acanthopores 

 occurring throughout the order Trepostomata. In addition, the type 

 species and most of the other Upper Ordovician species contain a 

 second, more distinctive type of acanthopore that extends through 

 only a part of the exozone width, is concentrated between zooecial 

 corners, and is ofifset from the zooecial boundaries toward the voids 

 of the zooecia resulting in inflection of the zooecial walls (see pi. 5, 

 fig. 2). In extreme development, such inflection gives a petaloid or 

 septate appearance to zooecia as seen in cross section in tangential 

 thin sections. The superficial likeness of the offset acanthopores to 

 radially arranged septa led Ulrich to term these acanthopores "pseudo- 



