DESCRIPTION OF 21 



PLATE XX. 



Page 



Fig. 1. SPIRIPER TENUIMARGINATA. 641 



1 a. View of the ventral valve. 

 1 b. Dorsal view. 



1 c. Profile view of the same shell. 



Fig. 2. SPIRIFER ROSTELLATUS. 641 



2 a. Dorsal valve of this species. 

 2 b. Ventral valve. 



2 c. Profile view. 



Fig. 3 & 2. SPIRIFER KEOKUK. 642 



3 a. Dorsal view, showing the area and foramen of the opposite valve. 

 3 b. Ventral view of a larger individual. 



3 c. Profile of a very gibbous specimen, where the beak of the dorsal valve 



is closed beneath that of the opposite valve. 

 3 d. Profile view of the specimen fig. 3 a. 



'2 d. Enlargement of surface, showing the radiating striae. 



. ,o I 



Fig. 4. SPIRIFER PSEUDOLINEATUS. 645 



4. Dorsal view of a specimen of medium size. 



Fig. 5. SPIRIFER NEGLECTFS. 643 



5. Dorsal view of a specimen distorted by pressure so that the valves are 



separated, showing the entire area of the ventral valve, the dorsal 

 valve being forced beneath the other along the cardinal line. 



Fig. 6. SPIRIFER SUBCUSPIDATUS. 646 



6 a. Ventral valve of the same. 



6 b. Dorsal valve and area of a specimen of ordinary size. 



Fig. 7. SPIRIFER LOGANI. 647 



7. Profile view. 



