BRACHIOPODA. 451 



Yarirty i:i]>h.js;i.J 



to 0. subcequata. The specimens illustrated by Mr. Walcott (loc. cit.) and occurring 

 abundantly in the upper beds of the Pogonip group -=Chazy group of New York, seem 

 to agree better with var. conradi Winchell. 



Formation and locality. In Minnesota this form is first met with at the top of the Trenton limestone, 

 where it is fairly abundant, but usually much crushed; thence it extends upward in the Trenton shales. 

 It has been collected at Minneapolis, St. Paul, Cannon Falls, Lanesboro and Fountain, Minnesota; Decorah, 

 Iowa. In Wisconsin it has been found in the "Lower Blue beds" and near the base of the " Upper Buff 

 beds " at Mineral Point, Janesville and Beloit; Dixon, Illinois. In the Glade limestone of middle Tennessee 

 the variety seems to be rare. It is probably this form of O. subcequata that also occurs in northern New 

 York and eastern Canada. 



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

 Mm. Reg. Nos. 186, 322, 336, 643, 3514, 4032, 5147, 5148, 7973-7977. 



Variety GIBBOSA Billings. 



I'LATE XXXIII, FIGS. 43-45. 



1857. Orthis gibbosa BILLINGS. Geological Survey of Canada; Keport of Progress for 1856, p. 296. 

 1859. Orthis gibbosa BILLINGS. Canadian Naturalist and Geologist, vol. iv, p. 434. 

 1892. Dalmanella gibbosa HALL. Palaeontology of New York, vol. viii, pt. i, p. 224. 



Original description: "About the size and shape of Orthis testudinaria, but with 

 both valves convex ; greatest width at the center or a little in front of the center of 

 the length, above which the sides are somewhat straight and converging to the 

 extremities of the hinge-line, the latter about one-sixth shorter than the greatest 

 width; the front margin very broadly rounded; almost straight or even slightly 

 sinuated in some specimens for one-third of the width in the center; front angles 

 well rounded; the ventral valve is depressed, pyramidal, most elevated at about one 

 line from the beak, which is small, pointed and but slightly incurved; a broad shallow 

 [often pronounced] mesial depression occupies the front of this valve, but disappears 

 usually at one-half the distance to the beak; cardinal area triangular at the base, 

 nearly at right angles to the plane of the margin, but curved over above, owing to 

 the backward projection of the beak. Dorsal valve exceedingly convex in most 

 specimens; greatest elevation about the center, often a barely perceptible broad 

 mesial elevation towards the front; cardinal area small, lying in the plane of the 

 margin; beak very small and scarcely projecting from the upper edge of the area; 

 the whole surface is covered with fine striae, which are about twice sub-divided; the 

 cast of the interior of the ventral valve shows that the muscular impressions were 

 bordered by strong lamellae extending downward, slightly converging at three lines 

 from the beak; in a specimen eight lines wide they were separated by a median 

 ridge with a broad base and sharp edge; width of large specimens, eight lines; 

 length, six and a half." 



Walter R. Billings has kindly presented to one of the writers specimens of 0. 

 gibbosa from the Black River limestone near Ottawa. These prove to be very closely 



