Review of Geinitz on the rocks and fossils of Nebraska. 173 
figures of internal casts illustrate clearly the broad furrowed car- 
oe area of Myalina, to which genus these shells appear to 
. : 
It is true Prof. King’s figures respecting internal casts of these 
shells, show a little projection between the beaks (as do some of 
our American species of Myalina), that might be mistaken for 
little ears,* but it is easy to see from perfect specimens showing 
that outside of the anterior margins of the shell, that these are 
not external ears, but the casts of a little cavity upon a kind of 
shelf or septum in the beaks, as stated by Prof. King. Prof. de 
Koninck mentions this little shelf and its concavity, in describ- 
ing the genus Myalina; and although a more or less varia- 
a 
ples of a shell that show, in one of the figures, some indica- 
Hons of a little anterior ear; but if it is really such, and not due 
forms, there is never a defined ear, or sinus, but on the contrary, 
the Margins of the valves fit closely there, and round up to the 
. nother mingling of distinct types under one generic name, 
1s Prof. Geinitz’s reference of such shells as the so-called Monotis 
‘peluncaria, and Pinna prisca, to the genus Avicula. The first of 
these types, it will be remembered, is exceedingly unlike the re- 
Rearly circular, plano-convex shells, with scarcely any —— 
ble obliquity, and almost destitute of proper alations at eit 
€xtremity of the hinge. These and other differences led Prof. 
King to separate the species speluncaria, from the genus { 
and to place it provisionally in the genus Monotis Bronn, in 
which he has, until recently, been generally followed, both in 
: country and in Europe. On comparing the American 
Precisely similar cavity exists be 
ilurian; and i 
A 
the Ton 
