DESCB.IPTIONS OF SPECIES. 425 



eral appearance of the shell is quite unlike M. suhqiiadrata , but the growth lines on 

 mature suhquadrata show that certain of its j^ounger stages must have closely resem- 

 bled the shell under discussion, eren suggesting the form commonly referred to M. 

 swallowi. I have therefore provisionally included this specimen under the common 

 title of 3f>/ali'?ia sniquadrata, though a diflferent course might have resulted from the 

 consideration of more complete material. 



Most of this material exhibits departures more or less extensive from the type 

 for which the name was original!}^ proposed, and I am not sure that they can appro- 

 priately be considered to belong to Shumard's species, except as well-marked varie- 

 ties. Two diverse types are shown by figs. 6 and 7 of PI. VIII. In one case it was 

 necessary, as indicated in the figure (T), to considerablj" restore the originally broken 

 specimen, but the lines of gi'owth have been followed as faithfully as possible, and 

 the shape represented is probably close to the original. 



Locality and horizon.— ^^w Juan region (stations 2200?, 2205?, 2340, 2341, 2342, 

 2344, 2345); upper portion of the Hermosa formation, and abundant in the Rico 

 'formation. 



Mtalina peeattenuata Meek and Hayden? 



1858. Mxialma (Mytelus) perattenuata. Meek and Hayden, Albany Inst., Trans., vol. 4, p. 77. 

 Permian: Smoky Hill Fork, Kansas and Missouri rivers. 



1859. Myalina {Mylilus) perattenuata. Meek and Hayden, Acad. Nat. Sci. Philadelphia, Prpo., p. 28. 



Upper Coal Measures: Smoky Hill River, Kansa.s. 

 1864'. Myalina perattenuata. Meek and Hayden, Smithsonian Cont. Knowledge, vol. 14, No. 172, p. 32, 

 pi. 1, figs. 12a, b. 

 Coal Measures: Opposite the northern boundary of Missouri on the Missouri River. 

 1866. Myalina perattenuata. Geinitz, Carb. und Dyas in Nebraska, p. 27, pi. 2, figs., 10, 11. 

 1873. Myalina perattenuata. Meek and Worthen, Geol. Surv. Illinois, Rept., vol. 5, p. 582, pi. 26, fig. 11. 



Coal Measures: Springfield, Illinois. 

 1891. Myalina perattenuata. White, U. S. Geol. Surv., Bull. No. 77, p. 28, pi. 4, figs. 13-15. 



Permian: Goodwin Creek and Military Crossing, Baylor County, Tex.; Camp Creek, Archer 

 County, Tex. 

 1895. Myalina perattenuata. Keyes, Missouri Geol. Surv., vol. 5, p. 118. (Date of imprint, 1894.) 



Upper Coal Measures: Kansas City, Mo. 

 1900. Myalina jteratlemiata. Beede, Univ. Geol. Surv. Kansas, Rept., vol. 6, p. 141, pi. 16, fig. 8. 

 Upper Coal Measures: Topeka, Ivans. 



After eliminating the types referred to Myalina stihqtiad/rata and to M. 

 Wyoming ensis several specimiens remain, unfortunately in not very good condition, 

 which resemble Meek and Hayden's species cited above. The shell, however, is a 

 large one, as the fact that the larger examples exceed 100 mm. in length will attest. 

 Some of these also approach M. exaspevata Beede. Doubtless, were the material less 

 imperfect, it would be possible to make a more satisfactory determination, and the 

 identification might not in every case be MyaMna perattenuata. 



Locality and horizon. — San Juan region (stations 2837, 2340); Rico formation. 



