430 THE PALEONTOLOGY OF MINNESOTA. 



[Ortliis (DInnrtliiM subquaclnitn 



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 striae. 



" Surface of both valves ornamented by moderately stout, radiating striae, 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; striae 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 are more or less interrupted in crossing some of the striae," 

 (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 0. meedsi is a development from 0. pectinella, var. sweeneyi, 

 in which a greater number of striae 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 striae is carried still further in 0. meedsi, var. germana, and in addition 

 to this, other features are introduced which link it closely to 0. 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 0. subquadrata is 

 thus probably completed. 



The following species, 0. proavita, is closely related to 0. subquadrata Hall. The 

 striae are simple, and it is rare to find interpolation and bifurcation taking place 



