PLATE XLVIII 



1'AGE 



Figs. 1-7. PLECTAMBONITES PISUM Ruedemann 253 



1. A pedicle valve less tumid than usual. X 1.25. 



2. Posterior view of same. X 1.25. 



3. Pedicle valve of normal dimensions. X 1.25. 



4. Brachial valve of an entire shell. X 1.25. 



5. Internal cast of pedicle valve showing casts of muscular and vas- 



cular impressions. X 1.25. 



6. Interior view of brachial valve. X 1.25. 



7. Pedicle valve with long cardinal line. X 1.25. 



Ordovician (Mohawkian), Rysedorph conglomerate, Ryse- 

 dorph Hill, Rensselaer County, New York. 



Figs. 8-11. PARASTROPHIA HEMIPLICATA Hall 269 



8. 9. Brachial and pedicle views of an entire specimen. 

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



Ordovician (Mohawkian), Lower Trenton portion of the Jack- 

 sonburg Is., near Newton, New Jersey. 



Fig. 12. ORBICULOIDEA LAMELLOSA (Hall) 234 



Exterior of a brachial valve. X 2. 



Ordovician (Mohawkian), Trenton Is., Middleville, New York. 



Figs. 13-15. ARTHROPOBA BIFURCATA Ulrica 224 



13. Segment natural size. 



14. Surface of upper part of original of Fig. 15 showing aged condition 



of zocecia. X 18. 



15. A small segment natural size and its upper part X 18 illustrating 



the youthful condition. 



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

 St. Paul, Minnesota. 



Figs. 16-18. CHRISTIANIA TRENTONENSIS Ruedemann 256 



16. Somewhat exfoliated pedicle valve. X 2. 



17. Brachial valve. X 2. 



18. Interior of brachial valve showing the quadrupal adductor scar 



divided by high vertical muscular walls. X 2. 



Ordovician (Mohawkian), Rysedorph conglomerate, Ryse- 

 dorph Hill, Rensselaer County, New York. 



Figs. 19, 20. ZYGOSPIRA EXIGUA (Hall) 273 



Pedicle and brachial valves illustrating the smooth exterior. X 2. 

 Ordovician (Mohawkian), Trenton Is., Watertown, New York. 



Fig. 21. CYRTOCERAS CAMURUM Hall 330 



Type specimen showing the rapidly curving shell. 



Ordovician (Mohawkian), Trenton Is., Middleville, New York. 



Fig. 22. ORTHOCEHAS ARCUOLIRATUM Hall 323 



Fragment of the shell, exhibiting the arched annulations. 



Ordovician (Mohawkian), Trenton Is., Watertown, New York. 



Figs. 23-25. ISOTELUS GIGAS DeKay 344 



23. A small but rather perfect specimen of the spinous form. 



24. A larger example of same. 



25. Aspinose form. 



Ordovician (Mohawkian), Trenton Is., near Trenton Falls, 

 New York. 



Figs. 1-7, 16-18 are after Ruedemann; 8-11, 13-15 are after 

 Ulrich; 12, 19, 20 are after Hall and Clarke; 21, 22 are after Hall. 



398 



