EXPLANATIONS OF PLATE 20 (11). 



Orthoceras medullare. page 353. 



Fig. 1. A fragment of a large individual, preserving several of the septa and a por 



tion of the outer chamber, together with considerable of the shell, which 



shows the character of the exterior surface. 

 Fig. 2. A section of the same individual, showing the transversely elliptical form and 



position of siphuncle. 



Orthoceras niagarense. page 356. 



Fig. 8. A view of the specimen described, showing the oblique undulations and several 

 of the sspta. 



Orthoceras annulatum. page 351 (47). 



Orthoceras nodocostum, M'Chesney, Iog. cit. 

 Fig. 4. A natural cast of the interior of a part of the outer chamber. The annulations 

 are very distinct and sharp, while the parallel encircling strias are very 

 obscure or scarcely distinguishable. The longitudinal ridges are unusually 

 well preserved, and give a nodose character to the annulations. 

 Fig. 5. A fragment of the septate portion of a specimen of this species, showing the 

 obscure impressions of longitudinal ridges with the annulations not strongly 

 elevated. 

 " 6. A view of the upper extremity of fig. 5, showing the broadly elliptical form of 

 the section and the position of the siphuncle. 

 This is the Orthoceros nodocostmn of M'Chesney. The species is in all respects identi- 

 cal with that in the Niagara group of New- York, which I have referred to O. annu- 

 latum of Europe. The New- York specimens occur in soft calcareous shale, and often 

 preserve the marks of obscure longitudinal ridges, interrupting the parallel transverse 

 striae, giving a very obscurely nodose aspect to the surface. This is shown in Pal. N. 

 Y., Vol. ii, plate 64, fig. 1 a. The same character is more distinctly shown in Murchi- 

 son's beautiful figure of this species, which in all its features corresponds with the 

 better preserved specimens in the Niagara group. The specimens from Illinois and 

 Wisconsin present a great variety in the degree of this marking, owing to the more or 

 less complete solution and removal of the shell and the nature of the enclosing mate- 

 rial. In many specimens there are no nodes preserved on the surface, and' though the 

 undulating transverse striae are rarely well preserved, they are nevertheless quite dis- 

 tinct in many of the specimens. Specimens also occur in which the undulating striae 

 and nodes are both well preserved. The specimen fig. 4 is the most extreme of any one 

 I have observed. 



