INVERTEBKATES. 345 



pointed above and rounded below, located near the middle of* 

 the posterior side, and curved parallel to the margin of the 

 valves. Height, measuring at right angles to the hinge, 1.73 

 inches; breadth, at right angles to height, about 1.66 inches; 

 convexity, 0.95 inch; greatest length, parallel to the umbonal 

 ridges, 2.12 inches. 



In size and general outline this shell resembles somewhat a species described 

 from Kansas, by Dr. Hayden and one of the writers, under the name of Mya- 

 lina avicuhides, {Proceed. Acad. Nat. Sci., Philad., May, 1860, p. 184*). Tt 

 differs, however, materially in the spirally curved character of the beak of its 

 left valve, and the less sinuate outline of its anterior side. It also differs in 

 having much stronger imbricating marks of growth, particularly on the left 

 valve. The Kansas species is likewise more nearly equivalve, and has its ante- 

 rior side more prominent, or somewhat lobed just under the beaks. 



Note. — Since the above was written, farther comparisons lead us to suspect 

 that this species is identical with 31. kansasensis, Shumard, {Trans. St. Louis 

 Academy Sci., i, p. 213), though none of our specimens show any traces of the 

 crenate lamallae mentioned in the description of that species. 



Locality and position: La Salle, Illinois; about the horizon of No. 11 Coal, 

 of Upper Coal Measures. 



Genus SCHIZODUS, King, 1844. 



(Ann. Mag. Nat. Hist., xiv, p. 313.) 



SCHIZODUS (sp?)- 



PI. 27, fig. 7. 



We only know this shell from imperfect casts, that give a rather unsatisfac- 

 tory idea of its specific characters. Since our figure was engraved, however, 

 we have found a better specimen, by breaking some masses of the matrix, than 

 that from which our drawing was made. From this it would seem that the one 

 represented by our drawing has lost portions of its anterior ventral margin, and 

 that our outline restoration of the broken posterior mai'gin is not exactly cor- 

 rect. Hence it is probable that our figure is turned so as to represent the beaks 

 and umbonal slopes as being too oblique, and the former too near the anterior 



* See also Paleontology of Upper Missouri, part 1, pi. ii, fig. 8 a, 8 6, 8 c, 8 a". 

 44 Sept. 24, 1866. 



