Miscellaneous. 485 
Note on the Shell-structure of certain Naiades. 
By C. A. Wurrr, M.D. 
Those who have given attention to fossil Lamellibranchiates can- 
not fail to have observed that, whenever the shell-substance is pre- 
served at all, it is universally thin, even in large specimens—too 
thin, indeed, to have given sufficient protection to the mollusk which 
it enclosed. It seems evident, therefore, that some portion of the 
shell-substance must have disappeared during the process of 
fossilization. Thus those fossil shells which possess a prismatic 
outer layer usually have this layer alone preserved, at least when 
they occur in calcareous strata. Numerous specimens of Myalina 
subquadrata from the Upper Coal-measures, and Inoceramus proble- 
maticus of the Cretaceous of Western Iowa, illustrate this well, in 
which the interior markings are uniformly obliterated, While 
lately examining the Naiades of the Iowa river I observed that they 
all possessed a prismatic outer layer of a character not distinguishable 
from that of Myalina subquadrata. The prisms are of about the 
same size in each; but those of the Inoceramus are nearly twice as 
large. 
The following species have thus far been found to possess the 
characters referred to:—Unio alatus, Say, U. cornutus, Barnes, U. 
crassus, Say, U. ebenus, Lea, U. plicatus, Say, U. rectus, Lam., U. 
securis, Lea, U. tuberculatus, U. ventricosus, Barnes, Alasmodonta 
complanata, Barnes, A, truncata, Say, and an undetermined species of 
Anodonta. 
This comprises all the genera (or subgenera) ound} in our region ; 
but it is not improbable that the prismatic structure is common to 
the whole family. The prismatic layer is of about the same thick- 
ness in all, the Anodontas and Alasmodontas being thinned at the 
expense of the inner or structureless layers; it is from half a 
millimetre to more than a millimetre in thickness, the epidermis 
resting directly upon it. Except in very old specimens, this layer 
constitutes the whole thickness of the shell for the width of a 
couple of millimetres from the margin ; and in very young specimens 
it exclusively occupies about half the space between the margin and 
the pallial line. 
The prismatic structure may be detected by the naked eye, and 
_ ean be well studied by the use of one of Tolles’s ?-inch triplet 
lenses. Under this magnifier the inner surface of the marginal 
border is seen to be finely granular, each granule being the con- 
vex end of one of the prisms which constitute the layer. These 
prisms extend through the layer to the epidermis without inter- 
ruption, reminding one, in their appearance upon fracture, of certain 
species of Chetetes. But viewing the prisms upon their ‘ends they 
are seen to vary in size much more than the cells of Chetetes do, 
and consequently to lack that uniformity in cross section presented 
by the coral. 
If our Naiades were fossilized under the same circumstances that. 
