NicKi.JSANDBAssi.EK.] GEOLOGIC DISTKIBUTION. ()7 



The Upper Helderberg fauna emphasizes the tendency J)egun in the 

 Niagara and strengthened in the Lower Helderberg — namely, the 

 diminution of the Trepostomata and the augmentation of the Crypto- 

 stomata. The Trepostomata are now of little importance; the Fenes- 

 tellidte far outweigh all other families and (constitute considerably over 

 one-half the fauna. 



The Hamilton fauna excels all others at present in th(> imraber of 

 described species and genera represented. Pro})ably a large amount 

 of synonymy still exists which time will eliminate. However, future 

 discoveries will probably disclose a number of new species more than 

 sufficient to offset the loss by reduction from synonyms. The Trepo- 

 stomata are the least important element, and are about ready to disap- 

 pear. Naturally they present a jumble of features which makes them 

 difficult to classify along with those of Eopaleozoic age. The Fistuli- 

 poridie have become wonderfully numerous in species and genera. 

 The Batostomellidaj are at their maximum; so also are the Fenestellida>. 

 The Falls of the Ohio has proved, probably, to be the most prolific 

 single locality for bryozoa in the world. The Cystodictyonidre are 

 also at their maximum. In fact, the Hamilton ma)^ be said to mark the 

 culmination of Paleozoic bryozoa. 



In the interval between the Hamilton and the Osage, the next for- 

 mation in which the bryozoa form a large element of the fauna, several 

 formations were laid down under conditions commonly unfavorable for 

 bryozoan life, though future collections ma}" materially alter this con- 

 clusion. The earliest of the Mississippian series, the Kinderhook, has 

 thus far shown but few bryozoa. The earlier of the Osage formations, 

 the Burlington, has also thus far proved poor collecting ground, but 

 the Keokuk and Warsaw, following, have an abundant bryozoan fauna. 

 At Warsaw, III., the Warsaw formation bears a close relationship to 

 the Keokuk fauna, but at other points the Warsaw seems more closel}^ 

 allied to the St. Louis, following it, than to the Keokuk, preceding. 

 In these formations the Cryptostomata are monopolists, but few Cyclo- 

 stomata and Trepostomata occurring. So it continues throughout the 

 Mississippian series. The Fenestellidas continue to be the predominant 

 family. As is the case in other tribes of the animal kingdom, when a 

 type of structure culminates, the family evolves a great many variations, 

 some curious, and even grotesque, ere it reaches its extinction. The 

 St. Louis, an almost solid limestone formation, has not been favorable 

 for the preservation of bryozoa. The Ste. Genevieve, which differs 

 from the St. Louis in having less limestone and more shale, has a con- 

 siderable bryozoan fauna, with the Fenestellidfe still much in excess of 

 the other groups. In the Chester the Fenestella type produces two 

 curious genera in form of growth, the Lyropora and Archimedes, both 

 of which must have been beautiful objects when living. 



