orgs aan eau a aaa, a Lk 
L. Waldo—New Position Micrometer. 49 
types have come down to us in unbroken lines some of which, 
to speak figuratively, were of remarkable tenuity. It is true 
there has been a dropping out of some of the earlier associated 
types and. an introduction of new ones as the epochs passed, 
but the lines of descent of the numerous types which have 
reached us unbroken, seem to be almost parallel, so little have. 
they changed with the lapse of time. So slightly divergent 
are these lines, considered as lines of differentiation, that i 
we bound them all by two imagivary straight lines, we shall 
have an evolutional parallax that would carry back the origin 
of these types to a period inconceivably remote. We must 
therefore conclude that their origin was, at least in some 
degree, saltatory ; but the real conditions under which they © 
originated must probably always remain ure. I have, 
however, elsewhere* suggested, that the differentiation of the 
Unionide took place under the influence of salt in the water 
in which they lived; but it is plain that this explanation 
will not apply to the case of the palustral and land mollusca. 
Art. V.—Description of a new Position Micrometer ; by 
LEonarp: WALDO. 
In the micrometer about to be described, both of these diffi- 
culties seem to be overcome. The screw bears, without any 
unguent, against an agate plane, and the pressure of both the 
Springs 1s practically constant in any position of the micrometer 
frame, which is liable to occur in actual use. In the sketches, 
/ . . . 
_88 represents the screw which is 19 cm. long, and 8 mm. in diam- 
* Bull. U, 8. Geol. Surv. Terr., vol. iii, p. 623. 
+I an principally indebted to Professors Lyman and Harkness, and the Messrs. 
Am. Jour. serie t Serizs, Vou, XX, No. 115.—Juxy, 1880. 
