120 



PALAEONTOLOGY OF THE UPPER MISSOURI. 



comes in contact with the so-called dorsal (properly ventral) side of the Scaphite; 

 while the truncated ends of the valves (Fig. 1, a, a, which represents the valves of 

 this specimen as it would appear if opened and flattened out) are directed forward 

 towards the aperture. 



The fact, however, to which we would call especial attention, is the occurrence of 

 a third piece or appendage, differing entirely in form from either of the two valves 

 already noticed, and, so far as we know, from anything hitherto found in connection 

 with any of these fossils. This third piece occupies a position between the two 

 valves as they lie together, partly opened; being nearer the extremity directed 

 towards the aperture of the shell, and exactly fitting between the valves, as if in its 

 normal position with relation to them. It is thin, and agrees so exactly, in color, 

 texture, and surface markings, with the two valves enveloping it, that it is impos- 

 sible to examine the specimen for a moment and entertain a doubt in regard to all 

 three of these pieces being parts of the same fossil. 



The appendage to which we allude differs entirely from the usual form of a Tri- 

 gonellite or Aptychus, as generally understood, and presents a very peculiar jaw-like 

 appearance. It consists of a single piece, with two thin rami or lateral expansions 

 extending backwards so as to present, when viewed on either side, the outline and 



Fig. 2. 



Fig. 3. 



Fig. 4. 



appearance of the annexed cut, Fig. 4 ; while Fig. 3 represents its upper side. Its 

 two lateral expansions, however (a, a of Fig. 3, and c of Fig. 4), have their upper 

 margins inflected so as to appear, as seen from above, to be thicker than they really 

 are. The position of this third piece between the two valves will be understood by 

 reference to Fig. 2, the right end of the figure being that directed towards the aper- 

 ture of the shell, and the lower or straight side being in contact with the so-called 

 dorsal side of the same. 



The presence of this third part or appendage would seem to furnish another 

 strong argument, if any were necessary, against the conclusion that these fossils are 

 the valves of Cirripedes, since its form is such that it can scarcely be regarded as 

 homologous with any of the external plates of those animals. Its form, if not 

 indeed its very existence, seems, we think, even more irreconcilable with the rather 

 generally accepted opinion that they are opercula. We can readily understand 

 how the two valves might be opened out and attached to a fleshy lobe, or some of 

 the softer parts of the animal, so as to perform the offices of an operculum ; but it 

 seems impossible to conceive how this third jaw-like piece, which is manifestly a 

 part of the same fossil, could be in any way connected with such an organ. In 

 addition to this, it may be added that many Ammonites are known to have the 

 aperture at maturity so remarkably contracted or modified by the development of 

 lateral appendages of the lip, that it appears almost impossible that they could have 

 been provided with an operculum. 



