CARBON I FKKOUS A<i I! C K K M I A N PERIOD. ;,;, 



Utter being somewhat more prominent than the other. Left Yalve convex ; anterior margin sometime* alight)/ 

 sinuous near tin* hinge abore ; posterior margin intenectlng the hinge at an obtain angle ; beak convex, extending 

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

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



Surface of both valve*, particularly the left one, ornamented with more or less distinct radiating cost*, which 

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

 pine-like prominences, formed apparently by the projecting lamina) of growth. Between each two of the principal 

 cosUe 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, 

 *//;//-,;//'</, Schlot. (sp.), and stated that we were aware it would not be easy 

 always to find characteristic differences by which certain varieties of these two 

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

 analogous cases, when- the \arieties of two closely allied, but variable species 

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

 tremely difficult 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. 

 ThN. we think, is the relations the Kansas shells bear to E. ttpehtncaria, 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. epelun- 

 i-nrin as arc 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 Ixan-:i-. \\ehavo never met with one presenting the peculiar 

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

 of E. sjtelnncaria, 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. Near 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.) 



l.iiiiiirrotiw Hawni, var. ovntn. 



(1'LATK II, Fig. 6, a, b.) 



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

 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 

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

 M. Garfortlieiutt, King, bnt its costs? are not so uniform, nor so spinons. We are Inclined to think it will prove to 

 be specifically distinct from K. llaicm ; bnt 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.) 



