94 PALEONTOLOGY. 



under the anterior ear is very deep, sharply defined, narrow, and directed 

 obHqnely backward toward the beak. Surface of left valve ornamented by 

 small, generally simple, depressed, radiating costse, between each tAvo of 

 which there is one, or on some parts two or three, smaller linear ribs, that 

 usually die out before reaching the beaks ; while small concentric wrinkles 

 are also usually more or less defined on the body-portion of the valve, so 

 as to impart a faintly tubercular appearance to the costse at the points 

 where they cross ; radiating costse on the ears, merely small raised lines, 

 rather distantly separated, and crossed at regular intervals by little sharply- 

 elevated lines, parallel to the free borders, so as to jiroduce a more or less 

 cancellated style of ornamentation. Surface of right valve (fig. 10 h) with 

 radiating costse (excepting one separating the posterior ear from the body of 

 the valve, and those on the anterior ear, which are larger than the others) all 

 small, and of nearly uniform size. Lines of growth very minute and ob- 

 scure on both valves ; though the rig-ht valve has some strong concentric 

 furrows. 



Height of one of the largest specimens, 0.88 inch ; breadth of the same, 

 0.89 inch ; length of hinge-line, 0.64 inch. 



This species is evidently allied to Aviculopeden occidentalis (zz Pecten 

 occidentalis, Shumard, = P. Cleavelandicus, Swallow), a very common and 

 widely-distributed species in the Coal-Measures of the Mississippi Valley. 

 It is a much thinner and more compressed shell, however, and always 

 without any traces of the subimbricating concentric lamellae of growth, 

 generally seen on well-preserved left valves of that species, and some- 

 times even rising as little vaulted scales on the costte, especially on those 

 of the anterior ear and near it. From A. Lyelli of Dawson, described 

 from the Lower Carboniferous rocks of Nova Scotia (which I am much in- 

 clined to think not distinct from A. occidentalis, Shumard), it differs in the 

 same characters distinguishing it from the latter. Compared with A. recti- 

 laterarius of Cox (sp.), it will be seen to diff'er not only in its deeply sinu- 

 ous posterior margin, and the consequent acutely angular character of its 

 posterior ear, but in having two sets of costit; ; that is, a larger and a 

 smaller set, generally alternating. It is probably most nearly allied to A. 

 Coxamis, Meek and Worthen, from the Coal-Measures of Illinois; with which 



