364 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



The relationship of the Cryptostomata to the Cheilostomata is 

 further suggested in the zoarial forms they assume and in the beauty 

 of the surface of the zooecia. In the typical Cryptostomata the 

 zoarium consists of two thin layers of zooecia growing back to back 

 into erect sword-shaped, ramose, ribbonlike or fan-shaped expansions. 

 In other Cryptostomata the zoaria form lacelike expansions consist- 

 ing of only a single layer of cells with the reverse side covered by a 

 dense layer of striated or minutely granulose tissue. In the remain- 

 ing sections of the order the zoaria are ramose with, the zooecia aris- 

 ing from a real or imaginary axis and opening on all sides of the 

 cylindrical stems. Usually the zoaria are continuous, but in some 

 of the bifoliate and ramose forms they are divided into segments, 

 articulating with each other. 



Most of the Cryptostomata can be identified from the zooecial 

 surface characters, but in some of them, particularly the bifoliate 

 and solid ramose species, thin sections are as essential as in the 

 Trepostomata. On account of their geometrical regularity of zooecial 

 form, thin sections of the Cryptostomata are often most beautiful ob- 

 jects under the microscope. 



The order commences in Early Ordovician times, reaches its 

 greatest development in the Devonian and Mississippian, and be- 

 comes extinct at the close of the Permian. Typical examples of the 

 order are illustrated in text figure 8. Many of the Ordovician 

 genera and species were described and illustrated by Ulrich in 

 1893 25 and Bassler in 1911, 26 the Silurian by Bassler in 1906, 27 the 

 Devonian by Hall and Simpson in 188T, 28 and the Carboniferous by 

 Ulrich in 1890. 29 



ORDER 5. CHEILOSTOMATA. 



The Cheilostomata, characterized by the closure of the aperture 

 by a chitinous lip or operculum when the polypide is retracted, in- 

 cluded most beautiful objects from an esthetic standpoint because 

 usually in this order the frontal wall of the zooecium is composed 

 of calcite assuming often the most delicate and sometimes bizarre 

 patterns. Until recently the differences in these patterns were relied 

 upon for the discrimination of genera and species, with the result 

 that a most unnatural classification prevailed. The calcification 

 of the frontal wall is only one of the functions of the bryozoan and 

 a natural classification should be based upon all the important fea- 



25 1893. Ulrich, E. O. Lower Silurian Bryozoa of Minnesota, Geology of Minnesota, vol. 

 3, pt. 1. 



28 1911. Bassler, R. S. The Early Paleozoic Bryozoa of the Baltic Provinces. Bulletin 

 77, U. S. National Museum. 



27 1906. Bassler, R. S. The Bryozoan Fauna of the Rochester Shale. Bulletin 292, U. S. 

 Geol. Survey. 



28 ] 887. Hall and Simpson. Corals and Bryozoa. Paleontology of New York, vol. 0. 

 29 1890. Ulrich, E. 0. Paleozoic Bryozoa. Geological Survey, Illinois, vol. 8. 



