F., B. Meek on the Family Pteriide. 219 
smooth, rounded, and without even the most obscure indication 
of an emargination, to represent the deep, sharply defined, byssal 
sinus of Humicrotis. 
I know nothing of the hinge, or of the microscopical struc- 
ture, of Monotis salinarius, the specimens at the Smithsonian 
Institution being firmly imbedded in the very hard brittle ma- 
rix, and not in a condition to show any traces of minute struc- 
ture. Dr. Carpenter, however, has examined a species—Avicula 
cygnipes, of Phillips—(which is unknown to the writer), supposed 
e congeneric with the type of Bronn’s genus, and finds it to 
possess the structure of the Pectenide, rather than that of the 
Aviculide. On examining thin sections of our Kansas shell, 
the type of the genus here described, by the aid of a magnifying 
power of about three hundred and fifty diameters, the prismatic 
ee P 
pearance of some species of [noceramus. They also have a small 
peculiar concave ear just in front of the beak of the left as well 
Margin in Aucella than in Humicrotis. Auet t 
Seem to bear similar relations to Humicrotis, that Posidonomya 
does to Monotis proper, as typified by Mf. salinarws. 
It is remarkable that, even in late European publications, we 
See the so-called Monotis speluncaria placed in the genus : 
There is still another small group of Jurassic shells represented 
by one species in our collection from the far West, for which I 
