366 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



0-7. Ilclopora spiniformis Ulrich. Ordovician (Lebanon) limestone of Central Ten- 

 nessee. 6. One of the jointed segments natural siae, and the lower portion of the 

 same, X 18. 7. Longitudinal thin section of the same species showing the short 

 immature region and the thickened mature zone. 



8-10. Cryptostomata with star shaped zoaria, two-thirds natural size, often mistaken 

 for fossil star fishes. 8. A five-rayed form (Evactinopora quinqueradiata Ulrich) 

 from the Lower Carboniferous (Burlington) limestone of Iowa. 9, 10. Two views 

 of a six-rayed form (E. scxradiata Meek and Worthen) from the same locality. 



17-19. Archimides, a characteristic Lower Carboniferous bryozoa, two-thirds natural 

 size, in which the lacelike cell-bearing zoarium similar to Fencstella is wound 

 around a solid spiral axis. Specimens (17, 18) with the celluliferous portion 

 broken away are most frequently found but occasionally more of the frond is pre- 

 served (19). 



20. Another characteristic Lower Carboniferous bryozoan (Lyropora) in which the 

 solid support is lyre shaped with the lacelike portion stretched between two sup- 

 ports. 



21-23. A typical member of the Cryptostomata (Arthropora simplex Ulrich) from 

 the Middle Ordovician (Black River) shales of Minnesota, showing several seg- 

 ments, two-thirds natural size, preserved in their natural position (21), the orna- 

 mentation of the zooecial surface, X22 (22) and a vertical section X12, illustrating 

 the typical internal structure of the Cryptostomata (23), namely, the short boxlike 

 immature zone with its hemisepta and the thickened mature zone with the zooecial 

 aperture (a) at the base of the vestibule (v). 



24-26. A bifoliate ribbonlike cryptostomatous bryozoan (RMnidictya mutaMlis Ulrich) 

 from the middle Ordovician shales of Minnesota. 24. A specimen, two-thirds 

 natural size. 25. The surface, X 12, showing the very regular arrangement of the 

 zooecia characteristic of the Cryptostomata. 26. Several zooecia still further en- 

 larged and illustrating the surface ornament. 



27. Streblotrypa hcrzeri Ulrich from the Lower Carboniferous rocks of Ohio. A 

 narrow ramose zoarium, natural size and X12, simulating Trepostomata externally 

 but having the internal structure of the Cryptostomata. 



28. Rhombopora ohioensis Ulrich from the Lower Carboniferous of Ohio, natural 

 size and X12, a representative of a characteristic Upper Paleozoic genus. 



29-30. Worthenopora spinosa Ulrich, from the Lower Carboniferous (Warsaw) lime- 

 stone of Illinois, showing possible relationship of the Cryptostomata to the Cheilo- 

 stomata. 29. A view of the bifoliate branch X9, illustrating the spinose margin. 

 30. Zooecia of the same, X28, showing the Cheilostomatous type of zooecia. 



passage of the eggs and the escape of the larvae, or, in other words, 

 the relations between the operculum and the ovicell; second, the 

 hydrostatic system and extrusion of the polypide, and lastly, calci- 

 fication and chitinization, or the nature of the skeletal part of the 

 animal. Therefore the least important of these functions has as men- 

 tioned before been almost invariably alone considered. These func- 

 tions are not difficult to determine in the recent forms, but in the 

 fossil species, where only the calcareous skeleton remains, it would 

 seem sometimes impossible to discover all of them. Fortunately the 

 form of the aperture indicates the hydrostatic function, the presence 

 of cardelles or projections on the apertural wall reveals the move- 

 ments of the operculum, and the nature and position of the ovicell 

 illustrates the function of reproduction. 



Function of reproduction. — A permanent classification of the 

 bryozoa is impossible at present, because each family is undoubtedly 

 characterized essentially by its larva and unfortunately the larval 

 form is known at present in only a few families. The fertilized eggs 

 of the bryozoan are transformed into embryos and these into larvae 



