SILURIAN AGE. 3 



Hall, in Foster and Whitney's Report, as a variety of Lingula antiqua, that we were 

 led to refer it to that species, not having specimens of the New York shell at hand 

 for comparison. Nor had we at that time within reach, specimens of the Wisconsin 

 form now made the type of the new genus Lingidepis. On comparison with speci- 

 mens of the latter, however, we find our shells agree so nearly with them that we 

 do not feel fully warranted in considering them distinct, though some slight 

 differences seem to be observable. They have the same general form and attenuate 

 beak, but appear to differ in having the beak of the larger valve more flattened 

 towards the pointed extremity ; still the species pinniformis seems to vary some- 

 what in this respect, and as our specimens are in a bad state of preservation we are 

 not sure these differences are constant. 



Should a comparison of better specimens show our shell to be distinct, it may 

 take the name Lingidepis dahotensis. 



Locality and position. Central part of Black Hills, Dakotah Territory. Pots 

 dam Sandstone, at the base of the Silurian system. (No. 1026a, type of description 

 and figure, Smithsonian Collection.) 



liin^ulepis prima. 



(Plate I, Fig. 2, a, b.) 



Lingula prima (Conead) Hail, Palseont., New York, I, 1847, 3, PI. i, Fig. 2, a, b. — Hall, Foster and Whitney's 

 Kept. Lake Superior, 204, 1851, PI. xxiii. Fig. 1. 



Shell small, ovate, rather gibbous, and comparatively thick ; rounded in front ; sides more or less convex in outline ; 



beaks obtuse and convex ; surface marked by obscure lines of growth, and more or less distinct radiating striae ; 



the latter most strongly defined on exfoliated surfaces- 

 length, 0.18 inch; breadth, about 0.14 inch; convexity (larger valve), 0.03 inch. 



Our specimens are generally worn, or more or less exfoliated, but as near as can 

 be determined they seem to agree with the above cited New York species. We 

 refer them to the genus Lingulepis, provisionally, not having seen the muscular 

 impressions, but believing it to be more than probable that most, if not all of the 

 older Palaeozoic species of this general form, will be found to possess the internal 

 characters of that genus. 



We would have suspected that these shorter and more oval specimens might be 

 the dorsal valves of the pinniformis, were it not for their smaller size, and the fact 

 that they are much more convex than those supposed to be the smaller valve of that 

 ■species from St. Croix Eiver. 



Locality and position, same as last. (No. 1027a, and 1027&.) 



Genus OBOLELLA, Billings. 



Synon.—Obolella, Billikgs, New. sp. Sil. Foss. Nov. 1861, 7, fig. 6, a, b, c, d. 



Aviciila 1 (desquamata), Hall, Pal. New York, I, 1852, p. 292, pi. 80, f. 3. 



Lincjulal (desquamata). Hall, Twelfth Kept. Regents' University, N. Y., Oct. 1859, p. 66. 

 Etym. — Diminutive of Obolus, the name of a small Greek coin. 

 Type. — Obolella chroniaiica, Billings. 



" Shell ovate circular or subquadrate, convex or plano-convex. Ventral valve 

 with a false area which is sometimes minute and usually grooved for the passage 

 of the peduncle. Dorsal valve either with or without an area. Muscular impres- 



