48 PROCEEDINGS OF THE NATIONAL MUSEUM. VOL, XXXIV. 
regarded as indication of different race, is regarded as definite indi- 
cation of the closest relationship between the several successive repre- 
sentatives of the genus. It is an expression of acceleration in growth 
coordinate with increase of size of individuals, and therefore may 
be ascribed to some evolutional force or forces operating upon the 
race, considered as a continuous series of genetically related indi- 
viduals, rather than to either heredity or environment. 
DESCRIPTION OF SPECIES. 
DALMANELLA LEONENSIS Hall, 1867. 
Plate Ii, ness i, 275,679) loys 24: 
A small orthoid shell of the type of Orthis testudinaria of the Or- 
dovician (ordinarily not over 12 mm. wide); valves subequal in 
thickness; the beak of the pedicle valve extends shghtly beyond the 
hinge line; the hinge is short; greatest width of shell is at or below 
the middle line; in the brachial valve the sulcus is not evident at the 
beak but gradually indents the shells along the median line; toward 
the front it is broad and shallow. The central part of the pedicle 
valve is slightly carinated from the beak forward. Neither the sul- 
cus nor the fold is sharply separated off from the low, curved general 
surface of the shell. 
The surface is marked by fine radiating, elevated lines, arising at 
the beak in 12 to 14 primary lines which are increased by implanta- 
tion of secondary lines on their sides; the secondary lines are gen- 
erally in pairs. A little further along in growth these secondary 
lines increase to nearly the size of the center ones, when a pair of 
tertiary lines appear on their sides, thus forming a spreading fas- 
cicle of lines which are subequal in size at the front. The lines curve 
backward at the cardinal angles. They increase to 20 or 25 each side 
of the middle at the front margin of specimens of ordinary size. 
One, two, or three concentric lines of growth occasionally appear. 
The type specimens used by Hall and specimens found in the same 
region identified with his figures appear all to have been thin shells, 
as indicated by the faint expression of their muscular scars and the 
appearance on molds of the interior in many specimens of lines run- 
ning over the place of the muscular scars. 
In the molds of the interior of brachial valves there is a distinct 
narrow flat groove (a narrow flattened ridge in the original) sepa- 
rating the muscular scars but inconspicuous beyond the anterior edge 
of muscular scars. There is a sulcus extending from between the 
scars forward to the front rim of the shell where it is generally 
broadened and shallow. The molds of the interior of the pedicle 
valves are nearly evenly rounded, showing little trace of central cari- 
nation, but the central part is more abruptly curved than the sides. 
