Oithls insculptit.l 
BRACHIOPODA. 
435 
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, tiie 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 localify.— Rare in the Trenton limestone and Trenton shales at Minneapolis and St. 
Paul; not nncoinmon in the lower portion of the Galena in many localities in Goodhue 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— Yf. H. Scofleld and the writers. 
Mus. Reg. Nos. 6752, 7806-78J2. 
Orthis insculpta Hall. 
1847. Orlhis insculpta Hall. Pala3ontology of New York, vol. i, p. 125, pi. xxxu, fig. 12, 
?1863. Orthis insculpta Billings. Geol. of Can., p. 167, (ig. 150. (Compare with O. hdlarugosa Conrad). 
1873. Orthis insculpta Meek. Palicontology of Ohio, vol. i, p. 99, pi. ix, fig. 1. 
1875. Orthis insculpta Millek. Cincinnati Quarterly Journal of Science, vol. ii, p. 40. 
. 883. Orthis bellarugosa Hall. Second Annual Report, N. Y. State Geologist, pi. xxxv. fig. 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 ZocaZii?/.— 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. 
Subgenus PLECTORTHIS, Hall. 
1892. Plectorthis, Hall. PaLTontology 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 anfl Hudson formations. While it retains 
the strong external ribs of the typical Orthis, these are not invariably simple {0. 
fissicosta Hall; 0. trijplicatella Meek; 0. wqiiivahis 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 lamellte 
and cardinal process is essentially the same as in Group I [Orthis calladis Dalman]: 
and the minute structure of the shell appears to be in precise agreement with that 
of 0. calligramma, though no evidence of tubulose costoe 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 suhquadrafa 
Hall and Orthis sinuala Hall]. 
