258 PALAEONTOLOGY OF ILLINOIS. 



striae. In some specimens the anterior ear is ornamented with 

 a few rather distinct radiating plications, crossed by smaller 

 snbimbricating marks of growth. Length, 3.63 inches ; breadth 

 from the anterior to the posterior margin, about 3.64 inches; 

 convexity of left valve, near 0.70 inch; length of hinge, about 

 2.48 inches. Some imperfect specimens in the collection mea- 

 sure as much as 5.50 inches in diameter. 



In the very distinct inequality of its valves, and the comparatively small size 

 of its posterior ear, as well as in some other characters, this fine species seems 

 to differ from the typical forms of Aviadopecten, being in these respects more 

 like Eumicrotis. It differs, however, from this group, in possessing a well de- 

 fined anterior ear, in the left valve, and a ligament area, without any carti- 

 lage pit or impression, as well as in having a merely laminated, instead of a pris- 

 matic structure.* It is probably related to a shell figured by McCoy, under the 

 name Malleus orbiadaris, in his Carb. Foss. Ireland, pi xix, fig. 2, though clearly 

 distinct specifically. Of course that excellent palaeontologist would not now 

 refer such a shell to Malleus. 



The specimens we have figured are not in a condition to show the muscular 

 impression, but we have indicated its position, form, etc., from others by the 

 dotted line in fig. 4c, pi. 18. The left valve, judging from some imperfect 

 specimens before us, is in some individuals proportionally more convex than 

 in that from which our measurements were taken. 



It is probable that Avicula magna, of Prof. Swallow, described in the Trans. 

 St. Louis Acad, of ScL, vol. ii, p. 98 (1862), is synonymous with this species. 



Locality and position: Monroe county, Illinois. Keokuk division of the 

 Subcarboniferous series. 



AVICULOPECTEN OBLONGUS, M. and W. 



PI. 18, fig. 3 a, 3 5. 



Avicula ohlonga, Meek and Worthen, October, 1860. Proceedings Acad. Nat. Sci., 

 Philad., p. 454. 



Shell small, nearly semicircular, moderately oblique; hinge 

 straight, and longer than any other part of the shell; pallial 

 margin broadly rounded; postero- ventral region more promi- 



* See American Journal of Science, xxxvii (second ser.), p. 218, and Palxont. Upper 

 Missouri, p. 50, 1865. 



