DESCRIPTION OF 



PLATE XXIV. 



Pu<re 



Fig. 1. PRODUCTUS OVATUS. 674 



1. Ventral valve of a specimen of this species. 



Fig. 2. PRODUCTUS TENUICOSTUS. 675 



o 



2 a. Ventral valve of this species. [ The anterior longitudinal folds or un- 

 dulations are not well shown in the figure.] 



2 b. Profile view, showing the extremely arcuate character of the shell. 

 2 c. View of the cardinal and upper portion of a ventral valve. 



2 d. Dorsal valve of this species. 



Fig. 3. PRODUCTUS MARGINICINCTUS. 674 



3 a. Ventral valve of a specimen of this species. [The figure represents the 



shell too broad, and the sides too much curved.] 



3 b. Profile view of the same, showing the arcuate form and prominence of 

 the marginal cincture. 



3 c. Dorsal valve of another individual. 



Fig. 4. SPIRIFER KEOKUK, var. 676 



4 a ( = 4 d of text). Dorsal view of a gibbous specimen, with the cardi- 



nal extremities rounded. 



4 b, c ( = 4 a, b of text). Ventral and cardinal views of a specimen, with 

 the hinge-line more extended. 



4 d. Cardinal view of a ventral valve, showing the area and foramen. 



Fig. 5. LlTHOSTROTION MAMILLARE. 667 



5 a. View of the calyces of a small group, showing the varying dimensions. 



5 ft. Vertical section of a portion of the mass, showing the columnar struc- 



ture where the cells are closely compressed ; the elevated columella 

 etc. 



Fig. 6. LlTHOSTROTION PROLIPERUM. 668 



6 a. A small group, showing the central cup, with the branches arising 



from proliferous budding. 



6 ft. A longitudinal section, which is a little on one side of the centre. 

 6 c. Transverse section enlarged. 



' 



[ IOWA PLATES.] 



