CARBONIFEROUS AGE. PERMIAN PERIOD. 5X 



the umbo by an oblique, rather deep, rounded depression. Surface ornamented by simple, rounded, unequal radi- 

 ating costse, which are crossed by exceedingly fine, regular, closely-arranged concentric strije, and a few stronger 

 marks of growth. 



Of the sixty or seventy radiating costse seen on the left valve, some nine or ten occupy each ear those on the 



anterior ear being a little coarser and more distinct than on the other. On the body part of the valve, about every 

 fourth or sixth one of the costae is a little larger than the others, and provided with a few distant, rather regularly 

 disposed, vaulted, scale-like projections, which may, on well-preserved specimens, sometimes assume the character 

 of short spines. Only the largest costse extend quite to the apex of the beak, while another series nearly reaches 

 it ; a third series dies out from one-half to two-thirds of the way up from the border, and a fourth extends 

 generally less than half way up. 



Height, from the ventral margin to the hinge, 0.83 inch; breadth, or greatest transverse diameter, 0.75 inch; 

 breadth of narrowest part just under the ears, 0.45 inch ; length of hinge, 0.49 inch ; convexity (of left valve), 

 0.22 inch. 



This species may be distinguished from any of those resembling it in other 

 respects, yet known to us from these rocks, by the rugose or subspinous character 

 of its largest costse, and its fine concentric striae. It bears some resemblance to A. 

 segregatus, McCoy (British Pal. Foss. pi. 3, E, fig. 1), but is a proportionally 

 narrower shell, has a shorter hinge, and more slender costse. It also probably 

 difi"ers in the possession of the fine concentric lines mentioned above, though Prof 

 McCoy's specimen was apparently not in a condition to have retained these, if it 

 ever possessed them. 



Named in honor of Prof Frederic McCoy, of Dublin, Ireland, the author of 

 the genus. 



Locality and position. — South Cotton-Wood Creek, Kansas. Permian beds. 



Family PTERIIDJE. (See page 27.) 



Subfamily PTERINIEN"^. (See page 28.) 



Genus MYALINA, Koninck. (See page 30.) 



niyalina aviciiloides. 



(Plate II, Fig. 8, o, 5, c, d.) 

 Myalina aviculoides, Meek & Hayden, Proceed. Acad. Nat. Sci. Phila. May, 1860, p. 184. 



Shell subtrigonal, higher than long, very convex, or subangular down the umbonal slopes ; anterior margin dis- 

 tinctly sinuous above the middle, thence descending with a slightly convex curve, nearly at right angles with the 

 hinge to the basal extremity, which is narrowly rounded. Posterior side compressed, its margin curving a little 

 forward above, or intersecting the hinge at right angles ; slightly convex and nearly perpendicular along the 

 middle, below which it curves obliquely forward to the abruptly rounded basal extremity. Hinge straight, nearly 

 equalling the length of the shell ; beaks very convex, subangular, and curving rather abruptly forward, so as to 

 become nearly or quite terminal. Surface having moderately distinct concentric marks of growth. 



Length, 1.48 inch ; height, 1.66 inch ; convexity of a left valve, 0.32 inch. 



This form may be readily distinguished from all the other species of the genus 

 known to us, resembling it in other respects, by its peculiar sinuous, or arcuate 

 front, and the extended or somewhat lobed character of its anterior margin under 

 the beaks. Young or undeveloped individuals are generally longer than high, and 

 have much more oblique umbonal slopes than adults. As the shell advanced in 

 size, however, the umbonal ridges curved down so as to stand nearly at right 

 angles to the hinge, and the valves became elongated in the same direction, so as 

 to make the height greater than the length. Usually the anterior margins of the 



