F. B. Meek on the genus Aviculopecten. 65 
acteristic of the various genera of the Aviculide, I was led 
to regard this group as most probably belonging to the Pec- 
tincdee and as holding in it a somewhat parallel position to that 
of Pterinea in the Aviculide. 
Some examinations, however, that I have recently had an 
opportunity to make, of the structure of several species of 
Aviculopecten apparéntly in a good state of preservation, col- 
lected by the Nebraska survey under the direction of Dr. Hay- 
den, seem to show that these shells have (when well preserved) 
an outer prismatic layer of shell as in the Aviculide, and not 
the structure of the Pectinide, 
The little experience I have had in making such examina- 
tions of the structure of fossil shells, has led me to the conclu- 
sion, that negative evidence should be received with great cau- 
tion in such cases, since these shells very often have the struc- 
ture entirely obliterated during the process of fossilization. In 
addition to this, it is probable that they are liable to have the 
very thin outer prismatic layer entirely exfoliated, so as to 
leave the surface markings tolerably well defined on the lamin- 
ated portion just within, which is then very apt to be mistaken 
for the original surface of the shell. Those I have just exam- 
ined, with one exception, appear to consist entirely of the thin 
outer layer, the laminated inner portion having been doubtless 
dissolved away after the shells were embedded in i 
They consist of the following species, and show what I am led 
to regard as a prismatic structure very clearly, viz., Pecten neg- 
lectus and P. Hawni Geinitz; both of which have all the external 
characters at least, of the genus Aviculopecten, and of course 
are not true Pectens; also Aviculopecten Comanus M. & W. and 
A. occidentalis Shumard, the latter being well known to have 
the flat striated cardinal plate and internal characters, as well 
as all the other peculiarities of the typical Aviculopectens. The 
others are as yet only known to me from what is believed to be 
the outer layer alone, which is too thin and fragile to bear pol- 
ishing, but when‘a fragment is carefully cleaned with a camel 
ir pencil, and examined under the microscope, by transmitted 
light, the structure alluded to can generally be seen with little 
difficulty, without any farther preparation, In these, I have not 
seen the flattened cardinal plate of the genus, but its absence 
is doubtless due to the fact that it was mainly or entirely com- 
posed of the wanting inner laminated portion of the shell.* 
