PLATE XXXV. 



PAGE. 



Figs. 1 and 2. AMBONYCHIA BELLISTRIATA Hall ^ 492 



1. Nearly an entire cast of a left valve. 



2. Anterior view of same, with the right valve supplied according to our idea of 



the species. Middle Galena, near Wykoff, Minnesota. 



. 



Figs. Sand 4. AMBONYCHIA PLANISTRIATA Hall 491 



Left side and anterior views of a specimen preserving the two valves partly 

 opened. "Lower Blue" limestone, Mineral Point, Wisconsin. 



Figs. 5 to 7. AMBONYCHIA AFFINIS, n. sp 492 



5 and 6. Lateral and anterior views of a well preserved fragment of the cast of a left 

 valve, showing very obscure concentric undulations, fine radiating stria; and 

 the sharply defined subrostral lobe. Galena, Carroll county, Illinois. 



7. An illy preserved cast of a right valve from the middle Galena at Weisbach's 



dam) near Spring Valley, Minnesota Survey Museum Reg. No. 8342. 



Figs. Sand 9. AMBONYCHIA AMYGDALINA Hall : 493 



8. The cast of an imperfect left valve. The specimen has suffered from pressure, 



reducing its diagonal diameter so that the anterior side curves too uniformly 

 into the base. 



9. Profile view, the right valve ideal Lower Galena, Goodhue county, Minnesota. 



Figs. 10 to 14. CLIONYCHIA LAMELLOSA Hall 494 



10. A small, but unusually well preserved cast of a left valve showing the impres- 



sions of the posterior adductor ami pedal muscles, pallial line and ligamental 

 area. This specimen is precisely like those which Hall named A. attenuata, 

 but its less quadrate shape is evidently the result of pressure. Trenton lime- 

 stone, Minneapolis, Minnesota. 



11. A left valve of the usual form from the "Lower Blue" limestone at Mineral 



Point, Wisconsin. 



12. Imperfect cast of a left valve preserving the muscular scars and a portion of the 



wide ligamental area. 



13. Anterior view of same, showing the impressions of the upper part of the margin, 



the form of the beaks and the pallial line running down from it. 



14. Cardinal view of same, showing scars of two small muscles (? pedal) behind the 



beak. One or both of these scars are present in all the Ambonychiidce. Lower 

 Trenton limestone, Beloit, Wisconsin. 



Figs. 16 and 16. CUONYCHIA NITIDA, n. sp 495 



Lateral and anterior views of a right valve, showing the form of the species and 

 the comparatively fine and regular cencentric lines. Trenton limestone, Min- 

 neapolis, Minnesota. Survey Museum Reg. No. 5099. 



Figs. 17 and 18. CUONYCHIA EBECTA Hall 496 



Lateral and anterior view of a nearly perfect right valve. Trenton limestone, 

 Minneapolis, Minnesota. 



Figs. 19 and 20. CUONYCHIA RHOMBOIDEA Ulrich. 496 



Lateral and anterior views of the specimen described. Trenton limestone, Min- 

 neapolis, Minnesota. Survey Museum Reg. No. 5526. 



Figs. 21 and 22. CUONYCHIA UNDATA Emrnons 497 



Lateral and anterior views of an excellent cast of a right valve. Middle Galena, 

 near Wykoff, Minnesota. Collected by Dr. Bobbins and now in the author's 

 cabinet. 



Figs. 23 to 26. BYSSONYCHIA INTERMEDIA Meek and Worthen 499 



23 and 24. Anterior and lateral views of one of the original types of this species. Galena 



limestone. 

 25 and 26. Cardinal and lateral views of a small but typical example from the middle 



Galena horizon, near Wykoff, Minnesota. 



Fig. 27. PROLOBELLA STRIATULA, n. gen. et sp 532 



The left side of a specimen, showing the form and surface markings. Middle 

 Galena, near Pleasant Grove, Minnesota. 



Figs. 28 and 29. MODIOLODON (?) OIBBUS, n. sp 522 



The left side of a specimen of the natural size and magnified four and one- half 

 times. Upper third of the Trenton shales, near Cannon Falls, Minnesota. 



Figs. 30 to 89. ARISTERELLA NITIDULA, n. gen. et sp 524 



30 and 31. Left and right sides of an entire shell, x 4.5. 



32 and .'{.'t. Lateral and cardinal views of the largest specimen seen, x 4 5. 



34 and 35. Lateral and cardinal views of an internal cast, in which the hight is less and 



the inciniality of the valves greater than usual, x 4.5. 

 36 to 38. Natural si/.e views of three shells. Middle third of the Trenton shales Chatfleld, 



Minnesota. 



39. Outline view of ;i stuill left valve from the same horizon at St. Paul. In this 

 shell the anterior end is unusually narrow. 



