52 PALAEONTOLOGY OF THE UPPER MISSOURI. 



valves below the most sinuous part are deflected from the umbonal slopes rather 

 abruptly inwards, so as to meet at an obtuse angle, or sometimes almost on the 

 same plane. 



Locality and position. Cotton- Wood Creek, Kansas, south of Kansas Eiver. 

 Permian beds. (Type 3961.) 



inyalina permiana. 



(PLATE II, Fig. 7, a, b, C.) 



Mytilus (Myalina) pcrmianus, SWALLOW, Trans. St. Louis Acad. Sci. Vol. I, March 8, 1858, 17. 

 Mytilui (Myalinal) concavus, SWALLOW, ib. 18. 



Shell obliquely subovate, or subtrigonal ; convex anteriorly, and cuneate behind ; beaks terminal, rather pointed, 

 and directed nearly horizontally forward at the points. Umbonal slopes prominent from the beaks down along the 

 front of the valves. Hinge straight, generally more than equalling half the greatest length of the shell, and ranging 

 at an angle of 60 to 65O with the oblique anterior margin ; cardinal area of moderate breadth, and distinctly striate. 

 Posterior margin rounding down with a broad sweep from the posterior extremity of the hinge, to the abruptly 

 rounded antero-ventral extremity ; anterior margin concave, or more or less arcuate in outline from the beaks to the 

 base, and very abruptly inflected from the prominent umbonal ridge of each valve. Surface marked by obscure 

 concentric strise, or nearly smooth. 



Length, from the beaks to the antero-basal extremity, 1.75 inch; length of hinge, 1.08 inch; breadth, 0.96 

 inch ; convexity, about 0.70. 



Prof. Swallow's descriptions of the two forms cited above, agree so nearly with 

 the specimens before us, that we can scarcely entertain a doubt in regard to their 

 identity, especially when we bear in mind that they came from the same beds, at 

 near the same locality. Our specimens also show various gradations between the 

 shorter and more elongate forms, so that we cannot believe a specific distinction 

 can be based upon these differences. 



This species is evidently very closely allied to a shell described by Dr. Isaac Lea, 

 of Philadelphia, from the Coal Measures of Pennsylvania, under the name of 

 Modiola wyomingensis (Jour. Acad. Nat. Sci. Phila. vol. II, 2d ser. p. 205, pi. xx, 

 fig. 1). Dr. Lea's figure seems to represent a slightly more oblique shell, with a 

 little longer hinge ; but as the species before us is variable in these characters, we 

 are prepared to believe the Kansas and Pennsylvania shells may possibly prove to 

 belong to the same species. If so, Dr. Lea's specific name will have to be adopted, 

 since his paper was published in 1852. 



The species under description may be distinguished from our M. perattenuata, by 

 its more robust appearance, broader form, and less attenuate beaks. It is probably 

 also a thicker shell than M. peraitemiata, though our specimens being all cast, we 

 are not sure this is the case. 



Locality and position. Trom Permian beds near the Smoky Hill fork of Kansas 

 River ; on Cotton-Wood Creek, and at many other localities south of Kansas and 

 Smoky Hill River, in northeastern Kansas. We think we have also seen it in the 

 Coal Measures below, in the same region. (Type 1153.) 



