NATURAL SCIENCES OP PHILADELPHIA. 245 



Contributions to the PALEONTOLOGY of Illinois and other Western States. 



BY F. B. MEEK AND A. H. WORTHEN, 



(Of the Illinois State Geological Surrey.) 



MOLLUSCA. 



LAMELLIBRANCHIA TA . 



Genus LITHOPHAGA, Bolten, 1798. 



(Lithodomus, Cuvier, 1817.) 



LlTHOPHAGA? PERTENU1S, M. & W. 



Shell slender, elongated, narrowing anteriorly, extremely thin, moderately 

 convex, in the central and anterior regions, more compressed and cuneate 

 behind ; posterior margin narrowly rounded in outline; anterior extremity 

 extremely short and very narrowly rounded ; basal margin straight along the 

 middle, and curving up gradually towards the extremities ; hinge line 

 straight, not exactly parallel to the base, and apparently about half as long 

 as the shell, passing imperceptibly into the posterior dorsal margin. Beaks 

 almost terminal, very oblique, and nearly obsolete; umbonal gibbosity 

 slightly raised above the hinge line. Surface smooth, or with only faint 

 traces of fine concentric stri e, and very obscure, undefined concentric undu- 

 lations. 



Length, 1*73 inch, height, 0'6"2 inch ; convexity, 0'50 inch. 



This species has very nearly the form of the following, but may be dis- 

 tinguished by its smooth surface, which never shows the distinct thread-like 

 concentric striae and regular wrinkles of that shell. As we know nothing of 

 the hinge and interior of these species, we merely place them provisionally in 

 the genus Lithophaga, from the similarity of their external characters, to the 

 recent L. lithophaga. Possibly their names should be written Modiolus lin- 

 gua/is and M. perlenuis, or more properly Volsella lingualis, and V. pertenuis, 

 as they may belong to that genus, and Modiolus and M>diola are merely sy- 

 nonyms with the older name Volsella. 



Locality and position. Warsaw, Illinois. Warsaw division of the subcar- 

 boniferous series. 



Lithophaga ? lingualis, Phillips (?) sp. 



Modiola lingualis, Phillips, Geol. Yorkshire, ii, p. 209, pi. v, fig. 2. 



The shell we have referred with doubt to Phillips' species cited above, 

 agrees with his figure in so many respects, that we are inclined to believe it 

 probably identical, though it may prove to be distinct on comparison of speci- 

 mens.*! It is an exceedingly thin shell, and ornamented by moderately 

 strong thread-like concentric strise, which, on the umbonal slopes, sometimes 

 form neat little ridges or undulations, while a few more irregular, distant 

 marks of growth, are seen on other parts of the surface. Some of the speci- 

 mens are much larger than that figured by Phillips, being nearly three inches 

 in length. The direction of the marks of growth on these larger specimens, 

 show that they were almost exactly like Phillips' figures, when of the same 

 size. They also show that the hinge line is long, straight, and bordered by 

 a slender marginal line within. 



Locality and position. Warsaw, Illinois. Keokuk division of subcarbonif- 

 erous series. 



* Tt is much to be regretted that many of the species figured by Trof. Phillips in his Geology of 

 Yorkshire, have not yet been fully and clearly described. 



18G5.] 



