496 THE PALEONTOLOGY OF MINNESOTA. 
[Orthis (Dinorthis) pectinella, var. Sweeneyi. 
OrvrHis (DINORTHIS) PECTINELLA, Var. SWEENEYI Winchell. 
PLATE XXXII, FIGS. 35—38. 
1881. Orthis sweeneyi N. H. WINCHELL. Ninth Report, Geological and Natural History Survey of 
Minnesota, p. 117. 
1892. Dinorthis sweeneyi HALL. Paleontology of New York, vol. viii, pt. i, pp. 196, 222, 228, pl. v, 
figs. 34-36. 
Original description: ‘Shell suborbicular, with a straightening along the hinge- 
line, and having the general aspect of Orthis pectinella, but with a shorter hinge-line. 
“The receiving [dorsal] valve is convex, with flattened lateral marginal areas 
and cardinal angles; costz coarse and simple, numbering about twenty-two, all of 
which continue to the beak except two or three on each side, which in passing from 
the margin in front of the cardinal angles, rather terminate on the hinge-line. The 
costz and the furrows, which have about the same width, are crossed by fine, 
crowded, concentrie striz; beak distinct, but not much elevated above the margin 
of the area; area slightly arched, but directed in the plane of the edges of the 
valves; area [foramen] triangular, equilateral, containing a simple tooth which rises 
to the apex, but is not developed so as to appear in the plane of the cardinal area, 
but is horizontally ribbed on either side. 
“The entering [ventral] valve is flat, with a little elevation at the beak and 
umbo, and a broad, sight concavity between the umbonal region and the front 
margin; coste the same as on the convex valve; beak small and more abrupt 
than that of the other valve; area low and flat, but of nearly the same hight 
as that of the other valve, with which it forms an angle of about 45°; foramen 
partially closed, but open below, broadly triangular. 
“The transverse diameter is seven lines in the single specimen belonging to 
the survey, and the perpendicular is six.” 
The specimen described, and others now before us, are essentially but dwarfed 
individuals of O. pectinella. Their shells, however, are smaller and thinner than in 
the typical form of that species, while the cost are usually simple, and the muscu- 
lar sears undefined. Since these features are constant in the specimens observed, it 
is considered advisable to recognize them as a variety of O. pectinella. 
Formation and locality.—Not rare in the upper portion of the Trenton shales at St. Paul, near 
Cannon Falls and Mineola, Minnesota; Decorah and McGregor, Iowa. 
Collectors—W. H. Scofield, F. W. Sardeson and the writers. 
Mus. Reg. Nos. 3510, 3520, 5001, 6791, 7768, 7769, 7785. 
