and Ichthyosiagonites of the Solenhqfen Limestone. 33 



my present ideas of the animals to which they belonged. I 

 perceived that the forms which naturalists had united in one 

 genus under the name of Trigo?iellites, Tellinites, Ichthyosia- 

 gotiites, and Lepadites, (words which are all synonymous,) be- 

 longed really to two distinct genera. One of these fossils is 

 not unfrequently found with an Ammonite-like shell, but 

 which has only an apparent likeness to the true Ammonite, 

 for it has no internal septa. In many of these Ammonite-like 

 shells there are found, near to their opening, two calcareous 

 plates resembling in appearance a bivalve shell. These must 

 in my opinion have belonged to the animal which inhabited 

 the Ammonite-like shell, and may have served as a kind of 

 operculum to it, or perhaps as an organ for mastication. I 

 formed this opinion by observing that whenever these two 

 fossils, apparently so dissimilar, are found near one another, 

 the right-hand side of the bivalve-like shell is uniformly of 

 the same length as the largest diameter of the external whorl 

 of the Ammonite-shaped fossil. Besides, I perceived that the 

 two parts which form this kind of operculum are in some 

 other points perfectly distinct in structure from any living bi- 

 valve shells, namely, the valves are not connected with liga- 

 ments, and have a sharp edge on the side where they unite ; 

 the other margin, opposite to this sharp edge, is a thick cal- 

 careous mass. The laminEs which are on the convex side of 

 these opercula have not, like other bivalves, a central point 

 round which they increase, but are placed somewhat in a dia- 

 gonal position, a circumstance which is never met with in a 

 real bivalve shell. 



Since I wrote the paper alluded to, I have observed a con- 

 siderable number of these fossils, all of which confirmed 

 the constant proportion of the diameters of the bivalve and 

 ammonite-like shell when found together. I remain, there- 

 fore, confident that they belonged to one animal, forming 

 quite a new type in the series of Mollusca, for which I have 

 proposed the name of Pseudammo7iites. 



Some naturalists have expressed the idea that the finding 

 both these fossils so frequently together was a consequence 

 of the animal of the Ammonite-like shell having eaten the 

 other. But if so, how does it happen that there is so con- 

 stantly a fixed proportion in their relative sizes, and why are 

 more than one pair never found in each shell ? Besides, had 

 the one served as food for the other, why are the apparent 

 bivalve shells always in a fine state of preservation, lying 

 parallel to each other? 



The other fossil, in shape somewhat similar to these oper- 

 cula, I consider to be an internal shell met with in a large 



Third Series. Vol.9. No. 51. Jw/y 1836. F 



