EXPLANATION OF PLATE NIV. 

 



Each figure (except 1 6) is represented tlie natural size of a medium 

 specimen. 



Fig. 1. — Spirifer IcHcis, Hall. 1. Ventral valve, viewed perpendicularly to 

 the plane of the margins, a. A patch of surface-markings. 16. A 

 portion of the margin of a large specimen, showing the rudimentary 

 plications, extending only a short distance from the margin. 



Fig. 2. — Sp. Icevis, H. a. Hinge-area, deltidium, and beak, of the ventral 

 valve, b. Dorsal valve, detached and lowered so as to expose the 

 area of the ventral valve above its cardinal margin. 



Fig. 3. — Sp. fimhriaius, Conrad. Ventral valve, viewed perpendicularly to 

 the plane of the margins. The dotted line on the left represents the 

 outline of the extreme of a conunon direction of variation. 



Fig. 4. — Sp. fimbrkdas, C. a. Beak and area of ventral valve, detached and 

 the beak tilted toward the observer, giving direct view of the area. 

 h. Perpendicular view of dorsal valve. 



Fig. 5. — Sp. Icevls, H. Side view. In general effect this is a restoration, 

 made upoii examination of a great number of distorted and imperfect 

 specimens. 



Figs. 6 and 7. — Sp. fimbrmhis , C. Cardinal and side views. 



Figs. 8, 9 and 10. Sp. crispus, Hisinger. Ventral, dorsal and side views 

 of a medium specimen. This is a reproduction of Hall's original 

 figures, 3 6 and 3 c of Pi. 54, Vol. 2, Pal. of K Y. The hinge-area 

 is, however, more produced than appears in specimens examined by 

 the author, a, on Fig. 9, is a small patch, slightly enlarged, of the 

 surface-markings 



Figs. 11a and 116. — Variety of Sp. crispus, showing the shortening of hinge- 

 line and ai'ea, greater elevation of beak, and lessening of plications. 



Figs. 12a and 6. — Sp. bicostatus, Vanuxem. — A copy of Hall's Fig. 4, PI. 

 54, 1. c, regarded as the extreme variety of the crispus type in the 

 direction in which Fig. 11 is intermediate ; the plications are obso- 

 lete, the area and beak high, the inequality in the relative convexity 

 of the valves extreme. 



Figs. 13a and 6. — Sp. crispus, His. Extreme variety in the opposite direc- 

 tion, toward Sp. sulcaius, in which are conspicuous the extended 

 hinge-line, small and low beak, low but still short area, distinct and 

 more numerous plications. 



