CARBONIFEROUS AGE. PERMIAN PERIOD. 55 



latter being somewhat more prominent than the other. Left valve convex ; anterior margin sometimes slightly 

 sinuous near the hinge above ; posterior margin intersecting the hinge at an obtuse angle ; beak convex, extending 

 but slightly beyond the hinge line. Kight valve nearly or quite flat ; beak flat, not projecting beyond the hinge ; 

 byssal sinus narrow, deep, or extending back parallel to the hinge to a point nearly under the beak. 



Surface of both valves, particularly the left one, ornamented with more or less distinct radiating costse, which 

 are usually separated by spaces three or four times their own breadth, and armed with regularly disposed vaulted, 

 spine-like prominences, formed apparently by the projecting laminae of growth. Between each two of the principal 

 costae from one to three or four much smaller radiating ribs or lines are usually seen, crossed by obscure con- 

 centric markings. (Hinge and muscular impressions unknown.) 



Length, 1.47 inch; height, 1.42 inch; convexity, about 0.40 inch. 



In first describing this species, we called attention to its close relations to E. 

 speluncaria, Schlot. (sp.), and stated that we were aware it would not be easy 

 always to find characteristic difi'erences by which certain varieties of these two 

 forms could be distinguished. Every naturalist, however, must have met with 

 analogous cases, where the varieties of two closely allied, but variable species 

 approximate, and, as it were, mingle together, so as to render it sometimes ex- 

 tremely difiicult to separate them ; while the normal forms of each are so clearly 

 distinct as to leave no doubt on the mind that they belong to different species. 

 This, we think, is the relations the Kansas shells bear to E. speluncaria, although 

 we are aware some of our friends entertain the opinion that they are not specifically 

 distinct. 



It is true, some specimens agree almost exactly with such varieties of E. spelun- 

 caria as are represented by figures 15, 17, 20 and 21, pi. xiii, of King's work 

 on the Permian Fossils of England ; yet out of hundreds of individuals, collected 

 and seen by us in Kansas, we have never met with one presenting the peculiar 

 lobed and sulcated posterior, so characteristic of the well developed normal forms 

 of E. speluncaria, such, for instance, as figures 5, 6, 7, 8, 9, 10 and 11 of plate xiii, 

 in King's work cited above. Again, none of our Kansas specimens, with a solitary 

 exception, has the beak of the right valve so gibbous, or near so elevated, as those 

 represented by the figures last above cited ; and in this single exception, the shell 

 differs so widely in other respects, that if not a monstrosity, we can but regard it 

 as belonging to a distinct species from that under consideration, as well as from E. 

 speluncaria. 



Locality and position.— l^estX the mouth of Smoky Hill fork, of Kansas River, 

 and at several places on the high country between there and Council Grove, as well 

 as on Cotton- Wood Creek, Kansas. Permian. (Type 3958.) 



Dumicrotis Hawni, var. oTata. 



(Plate II, Fig.^, o, b.) 



This variety differs from the typical forms of M. Hawni, in being more compressed, and more ovate in outline, 

 its diameter from the hinge to the ventral margin being proportionally greater; while its hinge margin is much 

 shorter, or not more than equalling about one-third, instead of one-half, the greatest breadth of the valves. Its 

 costse are also usually more distinctly defined, and its beak rather more pointed. In some respects it resembles 

 M. Garforthensis, King, but its costse are not so uniform, nor so spinous. We are inclined to think it will prove to 

 be specifically distinct from E. Hawni ; but as we are not clearly satisfied that this is the case, we merely name it 

 for the present as a variety of that species. 



Diameter, from hinge to ventral margin, 1.54 inch ; breadth, 1.43 inch ; convexity of left valve, 0.28 inch. 



Locality and position. — Near Cotton-Wood Creek, south of the Santa Fe Road, 

 Kansas. Permian. (Type 1157.) 



