PLATE XXIII 

 BILOBITES ACUTILOBUS RINGUEBERG (PAGE 400) 



FIGURE 1 . Outline of specimen from Niagara Group, Lockport, 

 New York. X 4. 



BILOBITES VERNEUILIANUS LINDSTROM (PAGE 400) 



FIGURE 2. Common elongate form from Upper Silurian, Gotland, 

 Sweden. X 4. 



BILOBITES VARIOUS CONRAD (PAGES 402-405) 



FIGURE 3. Dorsal view of youngest individual observed ; showing 

 inception of radiating striae and concealment of hinge-areas. X 18. 



FIGURE 4. Profile of same ; showing depth and extent of both 

 valves. X 18. 



FIGURE 5. Hinge view of preceding. X 18. 



FIGURE 6. Dorsal side of specimen ; showing beginning of anterior 

 marginal sinus. X 18. 



FIGURE 7. Profile of same, x 18. 



FIGURE 8. Posterior view of same. X 18. 



FIGURE 9. Dorsal view of specimen, figure 15 ; showing conceal- 

 ment of ventral area. X 9. 



FIGURE 10. Ventral view of same; showing dorsal area. X 9. Com- 

 pare this with dorsal view of larger specimen, figure 21, in series. 



FIGURES 11-26. Series of specimens; seen from dorsal side; exhib- 

 iting observed stages of growth, variation and development of hinge, 

 hinge-area, and marginal sinus. X 4. 



FIGURE 27. Interior of ventral valve; showing teeth, muscular im- 

 pressions, minute concave plate in apex of fissure, and arrangement of 

 punctse between nodes and ribs, x 6. 



Lower Helderberg Group, Albany county, New York. 



BILOBITES BILOBUS LINNI (PAGE 400) 



FIGURE 28. Outline ; showing characteristic form of this species as 

 occurring in Upper Silurian of Gotland, Sweden. 



