PLATE XLII 



PAGE 



Figs. 1, 2. STREPTELASMA PBOFUNDUM (Conrad) 199 



1. An entire specimen with a portion broken away to show the great 



depth of the body cavity. 



2. View of the body cavity. 



Ordovician (Mohawkian), Black River (Platteville) Is., 

 Mineral Point, Wisconsin. 



Figs. 3-5. TETRADIUM CELLULOSUM (Hall) 202 



3. Section in the rock cutting the tubes longitudinally and showing 



their arrangement in bundles. 



4. Portion of two bundles, enlarged. 



5. Cross-section in rock, exhibiting the arrangement of the tubes and 



the occurrence of the characteristic four septa. 



Ordovician (Mohawkian), Black River Is., Watertown, N. Y. 



Fig. 6. CAMAROCLADIA BUGOSA Ulrich 196 



Rock fragment composed almost entirely of the branches of this 

 organism. 



Ordovician (Mohawkian), Black River (Decorah) shales, 

 St. Paul, Minnesota. 



Figs. 7, 8. ORBIGNYELLA WETHERBYI (Ulrich) 214 



7. Vertical section exhibiting the characteristic curved diaphragms. 



X 18. 



8. Tangential section showing the angular tubes and the granular 



walls. X 18. 



Ordovician (Mohawkian), Black River (Platteville) Is., 

 Minneapolis, Minnesota. 



Figs. 9-12. ZYGOSPIRA RECUBVIBOSTRIS (Hall) 272 



9. 10. Brachial and pedicle valves of an entire specimen. X 2. 

 11, 12. Anterior and lateral views of the same. 



Ordovician (Mohawkian), Trenton portion of the Jackson- 

 burg, Is., Jacksonburg, New Jersey. 



Figs. 13-15. HELICOTOMA PLANULATOIDES Ulrich 300 



Three views of an entire silicified specimen. 



Ordovician (Mohawkian), Black River (Lowville) Is., Mercer 

 County, Kentucky. 



Figs. 16, 17. HELICOTOMA VERTICALIS Ulrich 301 



Basal and lateral views of a cast of the interior. 



Ordovician (Mohawkian), Black River (Lowville) Is., High 

 Bridge, Kentucky. 



Figs. 18-20. CTENODONTA GIBBEBULA Salter 277 



18. Lateral view of a large right valve. 



19. Cardinal view of the same. 



Ordovician (Mohawkian), Stones River (Murfreesboro) Is., 

 Murfreesboro, Tennessee. 



Fig. 1, 2 are after Winchell and Schuchert; 3-5 are after Hall; 

 7, 8, 18-20 are after Ulrich; 9-12 are after Weller; and 13-17 are 

 after Ulrich and Scofield. 



392 



