BRACHIOPODA. 425 



Orthis (Dinort)iisl pectinella. 1 



subconvex near the beak, with flat sides and a broad depression along the center, 

 which is distinct in front; ventral [dorsal] valve regularly convex, most prominent 

 in the center; beak extending only to the cardinal line; surface marked with from 

 twenty-two to thirty prominent, rounded radii, which are equal to the spaces 

 between; radii simple, or bifid and trifid towards the margin, crossed by small 

 [closely crowded] elevated concentric [growth] lines." 



In the interior of the ventral valve the dental lamellae are well developed and 

 unite with the outer elevated margin of the elongate-quadrate muscular area. 

 Within this area, occupying the central portion, are the elongate adductors divided 

 by a very faint median elevation. These scars are surrounded laterally and anteri- 

 orily by the diductors, while the adjusters lie outside of the latter at the base of the 

 dental lamellae. Genital spaces faintly indicated on the lateral area, each side of 

 the muscular depression. The apical third of the delthyrium has a convex deltidium. 

 Surface of both valves near the anterior margin more or less strongly marked by 

 plications, each centrally sulcated and opposite the depressions of the outer surface. 



Interior of dorsal valve with a well defined, bilobed and striated cardinal process 

 occupying the apical portion of the delthyrium; immediately underneath it is a 

 a low, rounded and short median septum separating the adductor scars, which are 

 rarely divisible into four impressions. Crural plates first form the walls of the 

 delthyrium and then extend into the interior as strong projections. 



Orthis pectinella is very constant in its characters throughout its geographical 

 distribution, and is restricted to the Trenton group. In Mercer county, Kentucky, 

 the largest growth and number of individuals is obtained, while in Minnesota the 

 tendency is towards dwarfing. The small size and neatness of the majority of indi- 

 viduals of this species led one of the writers to regard them as a distinct form, to 

 which the name 0. sweeneyi was applied. Recently, however, large specimens show- 

 ing the interior have been discovered, and these prove to be 0. pectinella. Since 

 more than two-thirds of the individuals occurring in Minnesota are smaller than 

 those obtained elsewhere, it is considered advisable to retain the name 0. sweeneyi as 

 a variety of the species. This new material has also shown that 0. charlottce Winchell 

 is founded upon an unusually convex ventral valve of 0. pectinella. 



Formation and locality Rare in the Trenton shales at St. Paul, Minneapolis and near Cannon Falls, 

 Minnesota; Decorah, Iowa. Common in Mercer county, Kentucky; Middleville, Trenton Palls, Turin, 

 Watertown and elsewhere in New York, Pennsylvania and eastern Canada. 



Collectors. W. H. Scofleld, C. L. Herrick, E. O. Ulrich and the writers. 

 Mus. Reg. Nos. 667, 7767, 



