DESCRIPTION OF 29 



' 









PLATE XXVIII. 



Page 



Fig. 1. SPIRIPER OPIMUS. 711 



1 a, b. Profile and dorsal views of a large individual. 



Fig. 2. SPIRIPER CAMERATUS. 709 



2 a. Dorsal view of a specimen above the medium size, having the extre- 



mities imperfect. 



2 b. Ventral view of the same. 



Fig. 3 & 4. PRODUCTUS COSTATUS, var. 712 



3 a. Ventral valve of a specimen preserving a greater number of spine- 



bases than usual. 

 8 b. Profile view of the same. 



4 a. Ventral valve of a strongly costate specimen with few spines, and 



a well-marked mesial sinus. 



4 b. Dorsal view of the same [the radiating costae and concentric wrinkles 

 are not properly shown in this figure] . 



4 c. Profile view, showing the curving row of spines which extend from 



near the beak. 



Fig. 5. ORTHIS ROBUSTA, 713 



5 a. View of the ventral valve, with the greater part of the shell exfoliated. 

 5 b. View of the dorsal valve, from which the striae are nearly obliterated. 

 5 c. Profile view of the same. 



5 d. Enlargement of the surface, showing the concentric striae. 











[ IOWA PLATES.] 5 



