ad 
TO GEOLOGY. 43 
better they 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 
JW. 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. Ihave never seen the E. car- 
dioides of Lamarck, the type and only species described by 
him. Cuvier says the Erycine approach the Mactre, and 
are but badly characterised. The same author says “the 
Almphidesme appear to approach the JVWactre, but they are 
too imperfectly known to have any distinctive character 
assigned to them.” Lamarck described sixteen species of 
Amphidesme 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 Amphidesme never 
have the angular cardinal tooth, so remarkable and striking 
in the Mactre, 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 (“par ce rapport singulier, d’avoir deux ligamens”), 
cannot be maintained, as many, perhaps all the Mactre 
have an external ligament, generally small, as well as an 
internal one; but in the JV. solidissima (Chemnitz) it is very 
perceptible, being, inalarge specimen, quite halfaninch long. 
The lateral teeth of the Amphidesmer also differ greatly. 
