34)4 Capt. Poi'tlock on the (/enus Cardiuia. 



self by a careful comparison of the casts of several living species 

 of the genus Unio with the fossil species of the stone-coal. The 

 internal casts of the true Uniones have, like Trigonia, a strong 

 anterior notch, and along the upper margin the impression of the 

 hinge-teeth is distinctly visible. In the fossil casts from the stone- 

 coal there are, on the contrary, two oblique furrows, the one an- 

 terior, the other posterior, which can only have originated from 

 widely separated hinge-teeth. I think that they woukl be better 

 placed in my genus Cardinia, which I have established from a 

 liassic species of the same type.^^ 



Again, under Unio acutus, pi. 33. fig. 5, 6, 7 , Sowerby states 

 that he had satisfied himself of the identity of the fossil with the 

 living genus Unio, by comparing a cast of the latter with several 

 fossil casts ; and the note of Agassiz upon this statemeiit is as 

 follows : — 



" What Sovt'erby here states of the generic identity of the so- 

 called Uniones from the stone-coal formation mth the species now 

 living in fresh Vv ater, merely proves that he recognised in the casts 

 both the principal characteristics of all elongated bivalves provided 

 with oblong hinge-teeth. But he has not thereby taken into ac- 

 count the great difference v>fhich the impression itself of these 

 teeth on the casts shows." In referring to Unio crassissimus, U. 

 Listen and U. hyhridus, the first of which is stated by Sowerby 

 to possess peculiarities in the great thickness of its shell and the 

 tile-like structvu-e of its surface, which might perhaps be elevated 

 into generic characters, Agassiz remarks, " These three species 

 belong to my genus Cardinia ; see the preceding note, and my 

 * Etudes critiques sur les Mollusques fossiles.^" 



Unio crassiusculus, pi. 185, and U. concinnus, pi. 223, are also 

 referred to Cardinia; but U. Solandri, pi. 517, and the several 

 Uniones figured in pis. 594 and 595, are stated by Agassiz to 

 belong principally to his new genus Pleuromya, for which he, as 

 before, refers the reader to ' Etudes critiques sur les Mollusques 

 fossiles.^ 



From these extracts then it is quite evident that Agassiz did 

 extend his genus Cardinia to the species of the coal as v»'ell as to 

 those of the lias, but whether he was correct in so doing can 

 scarcely be determined from the notes in question, as the actual 

 definition of the genus Cardinia is not given in them. This de- 

 ficiency Mr. Strickland can doubtless supply, and by placing the 

 characters of the several genera here alluded to in comparison 

 with each other, determine whether the shells of the coal forma- 

 tion can be classed in the genus Cardinia, or should form the 

 type of another new genus. 



