PLATE XXI. 



(Figures 1-29 by !l. P. Whitkikm).) 



Leg-end : A- Delthviiuui. c. Ci-uni. 



D. Deltidiuin. s. Median septum, 



t. Teeth. s'. Callosity in the <leltliyiiuin. 



d. Dental plates. x. Crural ridges, 



j. Cardinal process r. Diductor scars, 

 b. Dental sockets. 



Genus SPIRIFER, Sowekhv. 



Pago 1. 



Spirifer Niagarensis, Coin-iul. 



Fig. 1. A medium sized specimen ; showing the radiate-lineate exterior. 



Fig. 2. The inferior of the cardinal portion of a brachial valve ; showing Ihe crural plates. 



Fig. 3. A portion of the interior of the pedicle-valve ; showing the elongate muscular area. 



Fig. 4. The median portion of the cardinal areas of conjoined valves; showing the deltidium, cardinal 



process and crural plates. X 3. 

 Fig. 25. The surface characters. X 5. 



Niagara group. Lockport, N. F. 



Spirifer plicatellus, Liiine. 



Figs. 6-8. Figures of typical specimens of this form ; the tirst from Sweden, the other fi'om the Island of 

 Gotland. Figure 28 is au enlargement of the surface of the latter. 



Spirifer kadiatus, Sowcrby. 



Fig. 5. A small individual with a few low lateral plications. This is, apparently, one of the passage forms 

 between the typical iS. plicatellus and the non-jilicate (S*. radiatus. Distinction between the two 

 forms is not possible among American shells. 



Niagara group. Locality f 

 Fig. 9. A partial cast of the interior ; showing the diductor scars. 

 Fig. 10. A cardinal view of a specimen partially retaining the deltidium. 



Niagara group. Western New York. 

 Fig. 11. A typical non-plicate adult retaining the deltidium. 

 Fig. 12. The interior of the pedicle-valve ; showing teeth and muscular scars. 

 Fig. 13. The interior of the cardinal poi-tion of the brachial valve ; showing the dental sockets and crural 



plates. 

 Fig. 26. An enlargement of the surface characters. 



Niagara group. Waldron, Indiana. 



Spirifer radiatus, Sowei-liy (?). 



Figs. 15, 16. Opposite sides of an internal cast of both valves ; showing the elevated cardinal area, and im- 



pi'essions of the long dental plates in figure 16. 

 Figs. 17, 18. Two views of a lai'ger internal cast; showing the elevated area, and faint median ridge on the 



brachial valve. 

 Fig. 14. The interior of a portion of the conjoined valves ; showing a low median septum in the brachial 

 valve. 



Niagara dolomites. Wisconsin, 

 This shell is narrower on the lateral slopes than the more typical forms of /S. radiatus, and differs 

 from them intei'nally in the presence of the slight median septum in the brachial valve. 



