FOSSILS OF THE COAL MEASURES. 579 



with broad, faint, irregular concentric wrinkles, and a set 

 of obscure stri.e of growth. Often there is also an entirely 

 independent series of parallel riduv<. rro<sing the umbonal 

 Y( ui(n, or sometimes the whole surface obliquely, and 

 partly, or sometimes almost entirely, obliterating the other 

 markings. (Hinge and interior unknown.) 



Diameter of the largest specimen found, from the hinge 

 to the pallial margin, 1.30 inches; oblique parallel ridges 

 uniformly numbering seven to eight in 0.20 inch. 



Although the species upon which the genus Placitnfqm* was founded 

 was from the Great Oolite, and we have no knowledge of the genus 

 having 4 ever been identified from older rocks, our specimens agree so 

 exactly in all their visible characters with the figures and description 

 of that group, that we can scarcely doubt the propriety of referring our 

 species to it. Indeed, if it were not for the curious parallel, oblique 

 markings seen on most of the specimens, it might even be confounded 

 with the typical species of the gen us f P. J"wre;m, of R?mer) ; and if we 

 are not mistaken, one of MORRIS and LYCETT'S figures shows some 

 traces of similar markings, though they cross the valves obliquely in 

 the opposite direction. 



On some of the specimens we observe small discs growing, which we 

 should think probably the under valve of the young of the shell under 

 consideration, were it not that the minute apex of each is sub-central, 

 while the beak of our shell seems to have been marginal at all stages of 

 growth. 



We know of no other shell from any of our palaeozoic rocks with 

 which this is liable to be confounded. 



Locality and position Springfield. Illinois ; upper part of the Coal 

 -ires. 



Genus SCHIZODUS, King. 

 SCHIZODUS AMPLUS, M. and W. 



PL 27, Fig. 6. 

 Schizodiis amplu*. MEEK and WORTHEX, 1S70. Proceed. Acad. Xat. Sci.. Phila., p. 41. 



SHELL attaining a large si/e. as determined from inter- 

 nal casts, moderately convex, oblong-suboval in outline, or 

 about one-fourth of its length longer than high; anterior 



