426 THE PALEONTOLOGY OF MINNESOTA. 



[Orthis (Dinorthis) pictlnella, var. sweeneyi. 



ORTHIS (!)INORTHIS) PECTINELLA, var. SWEKNEYI 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. Palaeontology of New York, vol. viii, pt. i, pp. 196, 222, 228, pi. v, 



flgs. 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; costae 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-liae. The 

 costae and the furrows, which have about the same width, are crossed by fine, 

 crowded, concentric striae; 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, slight concavity between the umbonal region and the front 

 margin; costae 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 0. pectinella. Their shells, however, are smaller and thinner than in 

 the typical form of that species, while the costae are usually simple, and the muscu- 

 lar scars undefined. Since these features are constant in the specimens observed, it 

 is considered advisable to recognize them as a variety of 0. 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. Scofleld, F. W. Sardeson and the writers. 

 Mus. Reg. Nos. 3510, 3520, 5001, 6791, 7768, 7769, 7785. 



