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Figs. 1-3. SPYROCERAS BILINEATUM (Hall) 325 



1. Apical portion of shell showing gradual development of the annulations. X 2. 



2. Fragment of small shell exhibiting surface markings. X 2. 



Ordovician (Mohawkian) Black River (Decorah) shales of Minnesota. 



3. Portion of shell illustrating appearance when broken out of limestone. 



Ordovician (Mohawkian) Sinuites bed at base of Martinsburg shale, 1 

 mile south of St. Thomas, Pennsylvania. 



Figs. 4-7. ORTHOCERAS JUNCEUM (Hall) 323 



4. Fragmentary shell showing convexity of septum and a section of the same 



illustrating the central portion of the siphuncle. 



5. A shell with the outer chamber well preserved. 



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



6. Another fragment with the outer chamber and a number of septa preserved. 



7. Outer chamber of another example, natural size. 



Ordovician (Mohawkian) Sinuites bed at base of Martinsburg shale, 

 1 mile south of St. Thomas, Pennsylvania. 



Figs. 8-10. AMPHILICHAS TRENTONENSIS (Conrad) 354 



8. Dorsal view of a nearly complete cranidium. 



9. 10. Anterior and lateral views of the same. 



Ordovician (Mohawkian) Trenton portion of the Jacksonburg Is., 

 Jacksonburg, New Jersey. 



Fig. 11. EOHARPES OTTAWAENSIS (Billings) 332 



Billings' illustration of a nearly complete example of this fine species. 

 Ordovician (Mohawkian) Trenton Is., Ottawa, Ontario. 



Figs. 12-14. CYPHASPIS MATUTINA (Ruedemann) 353 



12. Dorsal view of cranidium of young specimen. X 10. 



13. Lateral view of same. X 10. 



14. Cranidium of older example. X 5. 



Ordovician (Mohawkian) Rysedorph conglomerate, Rysedorph Hill, 

 Rensselaer County, New York. 



Fig. 15. CALYMENE SENARIA (Conrad) 357 



Posterior view of pygidium illustrating the usual part of the trilobite 

 found. 



Ordovician (Mohawkian) Trenton portion of Jacksonburg Is., Jackson- 

 burg, New Jersey. 



Fig. 16. TRIARTHRUS FISCHERI (Billings) 341 



Type specimen. A glabella with the fixed cheeks. 

 Ordovician, Chazyan of Newfoundland. 



Figs. 17, 18. TRIARTHRUS BECKI (Green) 342 



Two specimens of average size showing the characteristic tubercles 

 along the median line of the axis. 



Ordovician (Cincinnatian) TJtica shale of New York. 



Figs. 19, 20. CRYPTOLITHUS TESSELATUS (Green) 335 



Dorsal and anterior view of the head, the portion usually found. 

 Ordovician (Mohawkian) Trenton portion of Jacksonburg Is., Jackson- 

 burg, New Jersey. 



Figs. 21-25. PROETUS LATIMARGINATUS (Weller) 352 



21, 22. Dorsal and lateral views of nearly complete cranidium. X 2. 



23. Dorsal view of same specimen. 



24, 25. A complete free cheek. Natural size and X 2. 



Ordovician (Mohawkian) Trenton portion of Jacksonburg Is.. Jackson- 

 burg, New Jersey. 



Figs. 26-29. ILLAENUS AMERICANUS (Billings) 349 



26, 27. Dorsal and front views of the original type. 



28. Pygidium of the same. 



29. A complete example. 



Ordovician (Mohawkian) Trenton Is., Ottawa, Ontario. 



Figs. 30-33. BUMASTUS TRENTONENSIS (Emmons) 351 



30. Cranidium of enrolled specimen with eyes preserved. 



31. Pygidium of another enrolled example. 



32. A complete although imperfect extended specimen showing the characteristic 



eight segments. 



33. Cephalon showing the lunate depressions. 



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

 Figs. 1, 2 are after Clarke ; 4, 5 after Hall ; 8-10, 15. 19-25 after Weller ; 

 11. 16, 26-29 after Billings ; 12-14 after Ruedemann. 



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