446 THE PALEONTOLOGY OF MINNESOTA. 



[Ortbls (Dalnianclls) sulKi'qu;it;i. 



1879, Orthis jugosa JAMES. The Palaeontologist, No 4, p. 31. 



1892. Dalmanella meeki HALL. Palaeontology of New York, vol. viii, pt. i. pp. 206, 224, pi. vc, fig. -'i. 

 1892. Orthis eorpidenta SAKDESON. Bulletin of the Minnesota Academy of Natural Sciences, 



vol. iii, p. 330, pi. v, flgs. 8-10. 



This variety is restricted to the upper portion of the Hudson River group and can 

 be distinguished from 0. testudinaria Dalman by the following characters: Attains 

 a larger size; valves thicker and more convex; cardinal areas wider and shorter; 

 strise coarser in all stages of growth, with a larger number terminating on the 

 cardinal lines; in the dorsal valve the muscular scars are more distinctly defined 

 and often much thickened and elevated around the margin. 



formation and locality. In the Hudson River group at Spring Valley, Minnesota, this variety 

 occurs in great numbers; also in the upper portion of the same formation at Oxford, Clarksville and 

 elsewhere in Ohio. 



Collectors. W. H. Scofleld, E. O. Ulrich and the writers. Also in the collection of Dr. C. II. 

 Bobbins, of Wykoff, Minnesota. 



Mus. Reg. Nos. 231, 236, 272, 4078-7985. 



ORTHIS (DALMANELLA) SUB^QUATA Conrad. 



PLATE XXXIK, FIGS. 30-36. 



1843. Orthis subcequata CONRAD Proceedings of the Academy of Natural Sciences of Philadelphia, 



vol. i, p. 333. 



1847. Orthis subcequata HALL. Palaeontology of New York, vol. i, p. 118, pi. xxxn, fig. 2. 

 1862. Orthis subcequata HALL. Geology of Wisconsin, vol. i, p. 42, flgs. 1-3, and p. 436. 



1880. Orthis minneapolis N. H. WINCHELL, Eighth Annual Report of the Geological and Natural 



History Survey of Minnesota, p. 63. 



1883. Orthis perveta HALL. Second Ann. Rept., N.Y. State Geologist, pi. xxxiv, flgs. (?16,) 17, 18. 

 1883. Orthis subcequata HALL. Ibidem, pi. xxxiv, flgs. 19-24. 

 1892. Dalmanella, subcequata HALL. Palaeontology of New York, vol. viii, pt. i, pp. 194, 207, 224, 



pi. vc. flgs. 6-11. 

 1892. Dalmanella perveta HALL. Ibidem, p. 224, pi. vC, flgs. 13, 14. 



Original description: "Semioval; valves ventricose, subequal; lesser valve with 

 a slight subangulated mesial furrow; larger valve prominent in the middle, with 

 flattened sides; radiating strise fine, closely arranged, unequal, rounded; cardinal 

 area rather wide; apex of large valve prominent, not profoundly elevated above the 

 opposite beak; the dorsal margin concave. Length, half an inch." 



Adult shells vary considerably in size; biconvex, sometimes as wide as long, but 

 generally attaining a greater breadth than length. In very young examples the 

 hinge-line is as long as the greatest width; while in large and obese individuals it is 

 only three-fifths of the breadth. Cardinal angles varying from acute to subacute 

 and sometimes slightly rounded; lateral and anterior margins broadly and evenly 

 rounding, the latter usually broadly, but slightly, deflected dorsally. 



Surface with fine stria?, tubulose, bifurcating about twice. . In the ventral valve 

 they are less numerous down the middle, but increase in size and have, at varying 

 intervals, very oblique, large opaniags. At the base of these perforations the stria' 



