Page 
243 
(The references to this plate, on page 240. should be to plate 41.) 
1. A small narrow form retaining the shell. 
2. A cast of a young individual. 
3. Cast of a valve of usual form. 
4. 5. Valves of small forms, having the cardinal extremities short and rounded? the latter 
figure showing the extension of the dental lamellae nearly to the front of the valve. 
6. Cardinal vievv of a specimen having a very high area and wide foramen.. 
7, 8. Forms in which the cardinal area and foramen are proportionally narrow. 
9. 11. Individuals of the prevailing form retaining the shell. 
10. Cast of a broader form. 
12, 14, 15. Specimens showing extreme variation of muscular impressioto. 
13,19,20. Individuals showing great extension of the cardinal extremities. Spirifera {Del- 
thyris ) perlata, Conrad. D. cuspidata, Hall. 
16. Cardinal view of the specimen, fig. 15, showing short thickened dental lamella?. 
17. ' Cast of a small individual, showing dental lamella? extending to the front of the valve. 
18. Interior of a specimen preserving a portion of the shell and showing the form of the mus¬ 
cular impression. 
(The figures on this plate are all ventral valves.) 
PLATE 42. 
Spirifera disjuncta .... 
