250 PROCEEDINGS OF THE ACADEMY OP 



In general appearance this shell approaches Edmondia ? compressa, of McCoy, 

 (Carb. Foss. Ireland, pi. 13, fig. 10,) but it is rather longer in proportion to its 

 height, and has much less prominent, and more compressed beaks, as well as 

 subangular, instead of rounded umbonal slopes. 



Locality and position. Lasalle, Illinois. Siliceous limestone of the Upper 

 Coal Measures. 



Genus CHiENOMYA, Meek, 1865.* 



CH/ENOMTA ? RHOMBOIDEA, M. & W. 



Shell rather small, short, moderately convex ; outline rhombic as seen in a 

 side view ; basal and dorsal marg'ns nearly straight and parallel ; the former 

 very abruptly curved upwards behind, and more gradually in front ; anterior 

 aide very short and truncated or a little rounded ; posterior side distinctly 

 truncated (obliquely) nearly the entire breadth or height of the valves, gap- 

 ing but not dilated ; dorsal margin less than the entire length of the shell, 

 and inflected so as to form a narrow but well defined escutcheon or false area. 

 Beaks narrow, or compressed antero-posteriorly, rather pointed, prominent 

 and incurved, nearly terminal or placed directly over the anterior margin. 

 Umbonal slopes oblique, very prominent near the beaks, but less so aleng 

 the central and postero-ventral regions ; anterior and ventral regions ab- 

 ruptly cuneate, with a very faint undefined impression extending from the 

 beaks obliquely backwards, towards the middle of the base, just in front of 

 the umbonal prominence. Surface of cast ornamented with small, regular 

 concentric undulations, with apparently very faint indications of very small 

 radiating striae. 



Length 1-17 inches ; height from ventral to cardinal margin, 0-80 inch, do. to 

 summit of beaks, 0-90 inch; convexity, 0-65 inch; gape of valves behind, 

 0-25 inch. 



Although this species seems to agree, in most of its known characters, with 

 the types upon which the genus Chanomya was established, it differs in being 

 a proportionally shorter and less widely gaping shell, while its beaks are much 

 more prominent and oblique. As we know nothing of its hinge or interior, 

 nor of its finer surface markings, it is only provisionally that we place it in 

 the genus Chcenomya. Possibly we should call it Allorisma rhomboidalis, or 

 Sedgwickia rhomboidalis. 



Locality and position. St. Louis Limestone, of Subcarboniferous Series. 

 Alton, Illinois. 



CHiENOMYA ? HYBEIDA, M. & W. 



Shell longitudinally oblong, moderately convex, somewhat arcuate ; dorsal 

 margin concave in outline, ventral border longer than the dorsal, and forming 

 a broad gentle curve nearly parallel to the dorsal outline, excepting a very 

 faint sinuosity in advance of the middle ; posterior side a little compressed 

 near the extremity, but rather distinctly gaping truncated or somewhat 

 rounded in outline ; anterior margin sloping forward from the beaks above, 

 and apparently narrowly rounded below. Beaks moderately prominent, some- 

 what compressed, and placed less than one-fourth the length of the valves 

 from the anterior extremity ; umbonal slopes not prominent ; flanks evenly 

 convex in the central region, and a little contacted anteriorly, so as to form 

 a very faint undefined depression from the bfjpts to the base. Surface (of a 

 cast)*showing small, obscure concentric ridges, which are most distinct and 

 regular along the posterior umbonal slopes, where they are abruptly deflected 

 upward at an obtuse angle ; anteriorly they are smaller, more closely arranged, 

 and deflected obliquely forward and upward. 



* In describing the genus Chxnomya in the Palaeontology of Up. Mo., p. 42, 1865, some doubtn 

 were expressed by me, in a foot note, whether or not it might bo identical with An'.hracomya, of 

 Salter, a description of which i had not at that time seen. Since seeing Mr. Salter's figures axti 

 description. I am entirely satisfied that these forms belong to clearly distinot genera. K. B. M. 



[Dec. 



