344 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1920. 



Order 1. CTENOSTOMATA. 



Zooecia gelatinous or chitinous with toothlike processes resembling a comb 

 closing the aperture when the tentacles are retracted. Range, Paleozoic to 

 Recent. 



Order 2. CYCLOSTOMATA. 



Zooecia calcareous and tubular with a circular aperture. Range, Paleozoic to 

 Recent. 



Order 3. TREPOSTOMATA. 



Zooecia calcareous and superposed upon each other so as to form long tubes 

 intersected by straight or curved partitions, and showing an axial, immature 

 zone and a peripheral, mature zone. Monticules or maculae of larger or smaller 

 cells distributed on the surface at regular intervals. Range, Paleozoic only. 



Order 4. CRYPTOSTOMATA. 



Gymnolaemata differing from the Trepostomata in that the primitive part of 

 the tube is usually much shorter and the passage to the mature region is more 

 abrupt. Triparietal gemmation. Probably the Paleozoic representatives of the 

 Cheilostomata. 



Order 5. CHEILOSTOMATA. 



Zooecia calcareous or chitinous with the aperture closed when the polypide 

 is retracted, by a chitinous lip or operculum. Range, Mesozoic to Recent. 



COLLECTION AND PREPARATION FOR STUDY. 



In view of their abundance in the sedimentary rocks and in the 

 recent seas, collecting specimens of bryozoa is a simple matter al- 

 though certain features of it should be mentioned. With regard 

 to the fossil forms, bryozoa are practically wanting in most sand- 

 stone strata, but beginning with the Ordovician there is scarcely a 

 limestone formation, especially if it has shale alternations, in which 

 they are not abundant. Generally the specimens are calcareous, and 

 in this condition are easily sectioned for study under the micro- 

 scope by the method mentioned later. Sometimes, however, they 

 are found silicified and the internal structure is, to a certain extent, 

 obliterated so that they can then rarely be successfully sectioned for 

 study. Such specimens, however, frequently preserve the surface 

 characters with great fidelity. In certain strata their substance has 

 been dissolved away leaving a perfect mold in the matrix. A gutta- 

 percha impression of this mold will often give a very satisfactory 

 idea of the exterior of the original fossil. 



The best specimens are usually obtained from the shales between 

 or just above or below limestone layers. The smaller forms may be 

 obtained free by carefully washing the shales and picking them out 

 from the debris. Some kinds of shales or clay will wash away better 

 if first allowed to become thoroughly dry. Others do better after 

 thorough soaking in water. 



Often the surface characters are obscured by the clayey matrix. 

 This may be removed by the use of caustic potash (KOH in stick 



