430 THE PALEONTOLOGY OF MINNESOTA. 
(Orthis (Dinorthis) subquadrata 
twice as high as that of the other valve, well defined, tapering rather rapidly 
towards the lateral extremities, arched with the beak, and directed backward and 
downward at decidedly less than a right angle to that of the other valve. Interior 
with muscular scars occupying a rather deep, bilobate impression extending nearly’ 
or quite to the middle of the valve and usually defined by a low ridge most distinct 
on each side; scars of adductor muscles small, separated by a mere trace of a raised 
line; those of the divaricator [diductor] muscles of moderate size, longitudinally 
striated and having their narrow posterior ends extending backward nearly to a 
small, triangular, transversely striated space [pedicle muscle scar] occupying the 
interior of the beak; those of the ventral adjustor muscles smaller and shorter than 
the divaricators [diductors] and situated nearly under the hinge teeth, which are 
moderately prominent, subtrigonal and oblique; vascular markings with their lateral 
divisions curving up backward and sending off several branches, while the other 
divisions extend forward and bifurcate so as to occupy the anterior region; anterior 
and lateral margins crenate within by very short striz. 
“Surface of both valves ornamented by moderately stout, radiating strizw, the 
posterior lateral of which curve so strongly outward that a few of them run out on 
the cardinal edge before reaching the lateral margins; striz of ventral valve nearly 
always increasing by bifurcation [some of them dividing two or three times], while 
those on the dorsal valve generally increase by the intercalation of shorter- ones 
between the larger. A few distant subimbricating marks of growth are sometimes 
seen towards the front and lateral margins; while, on perfectly preserved specimens, 
the radiating striae may sometimes be seen to be roughened by minute, elevated 
concentric lines, that aré more or less interrupted in crossing some of the striz.” 
(Pal. Ohio, vol. i, p. 94.) 
This species is so well known that a comparison with other related forms is 
unnecessary. 
It has been shown that O. meedsi is a development from 0. pectinella, var. sweeneyi, 
in which a greater number of strive have their origin at a much younger stage of 
growth. Moreover, as the tendency is to equalize the space between the ridges, new 
ones are continually added, so that these species at maturity are quite distinct. The 
multiplication of striz is carried still further in O. meedsi, var. germana, and in addition 
to this, other features are introduced which link it closely to O. subquadrata. The 
latter has all the characters of the former more strongly developed, which is due to 
its greater size, and the line of development from 0. pectinella to O. subquadrata is 
thus probably completed. 
The following species, 0. proavita, is closely related to O. subquadrata Hall. The 
strie are simple, and it is rare to find interpolation and bifurcation taking place 
