PLATE XXIII 



BILOBITES ACUTILOBUS Ringueberg (Page 400) 



Figure ] . — 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) 



Figures. — 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 Linne (Page 400) 



Figure 28. — Outline; showing characteristic form of this species as 

 occurring in Upper Silurian of Gotland, Sweden. 



