Lozver Silurian Fauna of Minnesota. 331 



surface ornamentation consists of fine, unequal radiating lines that bi- 

 furcate three or four times, and of rounded inconspicuous growth lines. 



From the upper part of the Cincinnati group in Fillmore county, 

 Minn. I have a scarcely distinguishable form from Butler county, O., 

 also. 



This species and the previous one might be considered as varieties of 

 O. testudinaria Dalman somewhat as are the forms in the Cincinnati 

 group of Ohio. I did at first think the last described species an extreme 

 form of O. emacerata Hall. Mr. Meek in his note on O. emacerata Hall 

 expresses such an idea concerning what is probably this form.* But after 

 looking over carefully the material collected last summer at Cincinnati, 

 O., I doubt the correctness of my first impression. 



Orthis rnultisecta Meek, and 0. meeki Miller are not represented in 

 Minnesota, and indeed the strata where the former should appear are 

 probably wanting. Even in the immature specimens of O. corpulenta the 

 acute cardinal angles and deep sinus of the Ohio specimens are wanting. 

 O. corpulenta seems to agree much more nearly with the form of O. tes- 

 tudinaria, found in the Galena formation of Minnesota, and differs from 

 this last mainly in size, and a few details on the interior, i. e., the muscle 

 scars are more nearly equal and more elongate in the former. This Ga- 

 lena form extends down from the Lingulasma bed to the Stictopora bed 

 (which perhaps equals Black River formation of New York). It agrees 

 closely in external form with the New York species from the Trenton, 

 but is perhaps smaller. Whether O. corpulenta or the following des- 

 cribed or some other is nearest O. testudinaria Dalman, I am at a loss to 

 say. 



ORTHIS ROGATA n. sp. or var. 

 Plate V, figures i, 2, 3 and 4. 



Compare O. testudinaria Dalman. 



Shell small, subcircular, width of a largest specimen 12 mm., length 

 II mm.; depth often equal to one-half the length. Hinge line equal to 

 two-thirds or three-fourths of the greatest width. Ventral valve strongly 

 and evefily convex (except near the mesial line on antiquated specimens 

 where the curvature may be abrupt.) Beak small, sharp and curved; area 

 small. Dorsal valve plane or convex with little or no sinus. Surface orna- 

 mentation as on O. corpulenta, but proportionately finer. The anterior pair 

 of muscle scars on the dorsal valve are much the larger. 



The specimen figured is the largest out of several thousand, and is 

 from the Orthisina bed at Berne, Dodge county, Minn. 



ORTHIS TERSUS, n. Sp. 



Plate v, figures 11, 12 and 13. 



Compare O. elegantula Hall, and O. hybrida Sowerby. 



Shells transversely elliptical, broadest towards the anterior ; len^gth 



of an average specimen 13 mm., breadth 15 mm., depth greatest just back 



of the umbo. Dorsal valve gently convex with a narrow furrow like 



sinus extending to, or nearly to, the anterior margin. Ventral valve 



strongly convex about the umbo, but less so toward the margin. No indi- 



•Palaeontology of Ohio, Vol. I, p. 110. 



