Mineralogy and Geology. 283 
For this species I would propose the name Aviculopinna Americana. 
_ As might be expected, the very thin substance of this shell is seen under 
the microscope to have a prismatic structure, like that of other types of 
the Aviculide. 
T avail myself of this opportunity to correct a few typographical errors 
feos in my remarks on the Nebraska fossils in this number of the 
ur 
a. 
® 
Vi 
on the C 
258 the writer has alluded to observations on the optical characters of 
the Chester chlorite made by Mr. Descloizeaux. Mr. Descloizeaux com- 
184, sixth line from top, strike out the ? after Rhynchonella 
orundophila 
clined to the plane of cleavage or the surface of the plates. An one good 
teriorly and green exteriorly. : 
5. Note on the optical characters of different micaceous minerals called 
: .D a, dated 
Paris, August 15, 1867).—(1.) In 18471 received from the vicinity of 
Sterzing, a foliated mineral having the plates a little concave, of whi 
‘olor and pearly luster, and well named perl-glimmer, which has the op- 
Neal axes distinctly united. as 
(2) The small specimen sent me recently by Mr. Dana as the original 
: , : : 
: 8 
white lamin mixed with some green clinochlore, and was regarded 
at that time as chloritoid. The lamine are very much macled, and the 
qretgence of the optical axes is accordingly very variable. I have found 
the red ray, at 20° C. with 
~ ‘MY specimen 2E—126° 24’; in another plate, =117° 30’, 
- Dana’s specimen 2E=109° 32’; “ a us 128° 38', 
The dispersion is very feeble, and it is rendered quite indeterminable by 
the numerous interlacings of the lamine. The plane of the optical axes 
iL a to be parallel to the longer diagonal. [This is the margarite of 
} _ Theclinochlore mixed with the margarite is macled, with distinct dis- 
| Persion (<u), and with strong double refraction. A plate gave me — 
( 2E—60° 48’ for the yellow ray. - : 
3.) In 1863 I received from Rammelsberg, under the name of Baryt 
vimmer [the CEllacherite of p. 256] a ed margarite from Pfitsch, 
Which is white and pearly, and has been analyzed . The 
