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in this, and in one other calcareous specimen, which I possess, the 

 whole substance. In some small specimens with which I have been fa- 

 voured by the Rev. J. R. Cleeve and Mr. Clarke, the cavities are filled 

 by a transparent calcedony; whilst the external opaque silicious sub- 

 stance yields every appearance to warrant the supposition of its being 

 the shell itself, thus changed by impregnation with silex ; for, although 

 the change is such, as to have given transparency to the mass, the 

 smooth shelly surface is evident. 



Plate X. Fig. 6 and 7, are the lateral and front views of a beautiful 

 fossil, which the late Mr. Martin, in his admirable work, Petrificata 

 Derbicnsia, PL 40, Fig. 1, 2, gives, rather hesitatingly, as a nautilites, 

 with this description " A fossil shell. The original a Nautilus. Invo- 

 luted, somewhat globose, im perforated, striated; the striae close, acute, 

 transverse, but oblique on the sides, and united by a single, narrow, 

 dorsal line, at the ambit. The mouth or aperture large, and somewhat 

 extended on each side. The form and situation of the dissepiments 

 unknown. Now and then found in our lime-stone, but not frequent." 

 I have rubbed down this fossil at the mouth, but without being able 

 to ascertain any thing with respect to either septa or siphunculus ; and, 

 of course, without being able to determine its genus. 



Plate X. Fig. 8, is a fossil shell of a very rare form. It is rather 

 globose, and marked with numerous transverse furcated lines, and three 

 deep grooves, which surround the whirl, and terminate on each side in 

 a small, but deep umbilicus. It possesses the character of Ammonites, in 

 having small and frequent foliaceous sutures, with, of course, sinuous 

 septa, and the character of Nautilus, in the outer turns, involving the 

 inner. With the habitat of this shell I am unacquainted. 



In another fossil from Italy, six times the size of the preceding, and 

 much flatter, an exact agreement with the preceding exists, except that 

 it is flatter. 



Plate X. Fig. 9, is a fossil, which is also of an ambiguous appear- 

 ance. The whirls are narrow, but deep, and marked with frequent 



