BRACHIOPODA. 435 



Ortliis insculptu.] 



and narrowly rounded medial septum. has its origin at the base of the apical thick- 

 ening, and upon each side are placed two pairs of adductor muscles, the posterior- 

 ones much the smaller. 



The larger specimens of this species occurring in the Trenton shales of Min- 

 nesota are difficult to separate from Orthis insculpta Hall of the Cincinnati group. 

 Commonly, however, the former is much smaller, has a less convex dorsal valve, 

 with more strongly defined muscular scars; the exterior, concentric, imbricating 

 growth lines are also more prominent. 



Formation and locality. Rare in the Trenton limestone and Trenton shales at Minneapolis and St. 

 Paul; not uncommon in the lower portion of the Galena in many localities in Gooclhue county, and at 

 Fountain, Minnesota; and Neenah, Wisconsin. Near the top of the Trenton at Decorah and McGregor, 

 Iowa. In the "Lower Blue beds" of the Trenton at Janesville, Wisconsin, and in the "Upper Buff beds" 

 at Rockton, Illinois. Also in the "Central limestone " of the Trenton near Murfreesboro, Tennessee. 



Collectors. ~W. II. Scofleld and the writers. 

 Mus. Reg. Nos. 6752, 7806-7812. 



ORTHIS INSOULPTA Hall. 



1847. Orthis insculpta HALL. Palaeontology of New York, vol. i, p. 125, pi. xxxa, flg. 12, 



?1863. Orthis insculpta BILLINGS. Geol. of Can., p. 167, flg. 150. (Compare with O. bellarugosa Conrad). 



1873. Orthis insculpta MEEK. Palaeontology of Ohio, vol. i, p. 99, pi. ix, flg. 1. 



1875. Orthis insculpta MILLER. Cincinnati Quarterly Journal of Science, vol. ii, p. 40. 



. 883. Orthis bellarugosa HALL. Second Annual Report, N. Y. State Geologist, pi. xxxv, flg. 22. 



1892. Htbertella insculpta HALL. Palaeontology of New York, vol. viii, pt. i, p. 222. 



It is peculiar that this prevalent species of the upper beds of the Hudson River 

 formation does not occur in Minnesota, since it is found in Wisconsin and Iowa. 



Formation and locality A. characteristic fossil of the upper beds of the Cincinnati group in the 

 Ohio valley; also at Wilmington, Illinois; Iron Ridge, Wisconsin, and Graf, Iowa. 



Mus. Reg. Nos. 7813-7815. 



Subgeuus PLECTORTHIS, Hall. 



1892. Plectoi-this, HALL. Palaeontology of New York, vol. viii, pt. i, p. 194. 



Original description: "This is a persistent form, which in American faunas, so 

 far as known, is limited to the Trenton and Hudson formations. While it retains 

 the strong external ribs of the typical Orthis, these are not invariably simple (0. 

 fissicosta Hall; 0. triplicatella Meek; 0. cequivalvis Hall, not Davidson; 0. jamesi Hall); 

 the cardinal area of the pedicle valve is comparatively low and the valves are sub- 

 equally convex. In the interior the character of the muscular scars, dental lamellae 

 and cardinal process is essentially the same as in Group I [Orthis culladis DalmanJ; 

 and the minute structure of the shell appears to be in precise agreement with that 

 of 0. calligramma, though no evidence of tubulose costse has been observed. In 

 Orthis jamesi, which is placed in this association, there is occasionally a deviation 

 toward the resupinate contour, exemplified in Groups IV and V [Orthis subquadrata 

 Hall and Orthis sinuata Hall]. 



