EXPLANATIONS OF PLATE 19 (10). 



Orthoceras orebescens. page 354. 



Fig. 1. Yiew of a large specimen, preserving the outer chamber and several of the 



septa ; the lower end showing the depth of the septa. 

 " 2. View of another individual, having part of the septa in the lower end removed, 



and exposing the sii)huncle. 

 " 3 A smaller individual, which preserves traces of the longitudinal ridges. 



Orthoceras columnare. page 351 (47). 



Fig. 4. A fragment preserving the filling of four chambers, which are very distant. 



" 5. A transverse section of the lower end of the preceding specimen. 



" 6. View of another specimen, preserving nine chambers, which are very irregular 

 in their distances. In the upper part there is a small piece of the shell re- 

 presented, showing the surface characters. 



" 8. A fragment of this species of smaller size, preserving essentially the same 

 characters. 



Orthoceras loxias. page 380. 



Fig. 7. Figure of specimen natural size, preserving about seventeen of the septa ; the 

 shell being partially preserved, or replaced by mineral matter on the other 

 parts of the surface. This species is not positively known in the Niagara 

 limestone, and is therefore omitted in the index. 



Orthoceras angulatum [?] page 353. 



[See remarks on this species in Supplementary Notes.] 

 Fig. 9. A fragment of this species of about six inches in length, preserving above 

 twenty septa and a part of the chamber of habitation ; from Racine, 

 Wisconsin. 



" 10. From an impression of the exterior of a specimen similar to fig. 9, and from 

 the same locality. 



" 11. From an impression of the exterior of a similar form, from Bridgeport, Illinois. 

 The specimen, at a point where it is one inch and five-eighths in diameter, 

 preserves above twenty longitudinal ridges in the semi-circumference, giving 

 more than forty in the entire circumference. The character of surface in 

 these impressions is precisely like that of 0. cancellatum, Hall, from the 

 Niagara group of New- York, and differs in no essential particular from the 

 minute surface markings of 0. columvare. 



