WAVERLY GEOUP SPECIES. 297 



ones generally dying out at various distances between the free margins 

 and the beak ; crossing all of these radiating costte, numerous, much 

 finer, more crowded, and more regular concentric striae, and a few very 

 obscure ridges and furrows of growth, may be seen. 



Breadth of largest specimen of this valve seen, 1.88 inches ; height, 

 1.53 inches; convexity, about 0.24 inch; length of hinge, 1.10 inches. 



The foregoing description and measurements are, as stated, made out 

 entirely from left valves. There are, however, in the collection, from the 

 same beds and locality, casts of some smaller right valves, that almost 

 certainly belong to the same species. (See fig. 66.) These are very 

 nearly, or, sometimes, quite flat, have the ears proportionally larger, so 

 as to make the hinge line as long as the breadth of the disc below, while 

 the sinuses under their ears are deeper and narrower (particularly the 

 anterior one*) than in the other valve. They also difier in having the 

 flattened disc smooth, or only showing minute, crowded, concentric striae, 

 with sometimes very faint traces of radiating lines. On the ears, how- 

 ever, this valve has the radiating costa3 nearly or quite as strong as those 

 of the other valve. 



The specimens of the left valve vary somewhat in form, some of the 

 smaller being proportionally narrower across the disc, and having the 

 ears more nearly equal (the anterior one being also more acutely angu- 

 lar) than that represented by our figure 5a. In other large specimens, 

 however, the proportional breadth is even somewhat greater than in that 

 we have figured. 



This species has very nearly the outline and surface markings of A. 

 Coxanus, Meek and Worthen, from the Coal Measures of Illinois and 

 Nebraska.! The right valve of the A. Coxanus is unknown, but its left 

 valve is so nearly like that of certain varieties of the form under consid- 

 eration, that there is little, excepting the much larger size and more 

 robust appearance of the Waverly shell, by which the two can be distin- 

 guished. Yet I have no doubts in regard to their belonging to distinct 

 species, not only from the great difference of size, but also on account of 

 the rather widely separated horizons at which they occur, the species of 

 this type of lamellibranchiate shells being usually more restricted in 

 their vertical range. In size, it agrees more nearly with Aviculopecten 

 rectilaterarius, Cox; but it difiers from that species in having a deep 



* This anterior sinus is not represented quite angular enough in our figure .56. 



t See Palaeontology Eastern Nebraska, published in Hayden's Nebraska Report of 

 1872, pi. 9, fig. 2a. 



