TO GEOLOGY, 43 



better tliey should. They there form a beautiful na 

 tural group. While in London last summer, Mr Gray 

 showed me in the British Museum a specimen of the 

 M. arctata, the habitat of which he was not before ac 

 quainted with, and which he informed me he had de 

 scribed in one of the scientific journals of London eight or 

 ten years previously as an Erycina. 



Mr Conrad, being now actively engaged in investigating 

 the fossils of some of our southern formations, may be able 

 to throw further light on this subject. 



Some European naturalists consider the Mactra donacia 

 (Lamarck) as an Erycina. I have never seen the E. car- 

 dioides of Lamarck, the type and only species described by 

 him. Cuvier says the Erycinw approach the Mactrce, and 

 are but badly characterised. The same author says &quot;the 

 JlmphidesmcB appear to approach the J[factr& 9 but they are 

 too imperfectly known to have any distinctive character 

 assigned to them.&quot; Lamarck described sixteen species of 

 JlmphidesmcB in 1819, and as many more have probably been 

 described since. I cannot understand how there can be any 

 difficulty in regard to this genus. The JlmphidesrycR never 

 have the angular cardinal tooth, so remarkable and striking 

 in the Mactrce, and this alone is sufficient to separate them. 

 The chief character by which Lamarck makes the division, 

 viz. the existence of an external as well as internal liga 

 ment (&quot; par ce rapport singulier, d avoir deux ligamens&quot;), 

 cannot be maintained, as many, perhaps all the tMactrm 

 have an external ligament, generally small, as well as an 

 internal one; but in the M. solidissima (Chemnitz) it is very 

 perceptible, being, in alarge specimen, quite half an inch long. 

 The lateral teeth of the Jlmphidemce also differ greatly, 



