444 CARBONIFEROUS LAMELLIBRANCIITAT \ . 



species to S. codellata. The two species agree perfectly both in .diagnosis 

 and figure, with the exception that the antero-inferior angle of the figured 

 specimen seems to be more cut away than in any other specimen that I have seen. 

 This may possibly be due to the imperfection in the original specimen. The 

 characteristic radiating ribs are different from those possessed by S. primseva, both 

 in character, grouping, and distribution. 



The shells described and figured both by de Verneuil and M'Coy {oji. cit.) 

 under the name *S^. prlmseva are without doubt examples of 8. costellafa. M'Coy's 

 description is very careful and accurate, the characteristic ribs being described at 

 considerable length. I am able to reproduce the specimen that he figured 

 (PI. L, fig. 13), which is that of an almost perfect cast, by the kindness of 

 Prof. McKenny Hughes, of Cambridge. M'Coy did not give a figure of the 

 exterior ; and, indeed, it is very rare to meet with examples having the test 

 preserved, owing to its fragility. M'Coy's figures are largely hypothetical, 

 especially the view of the hinge-line and umbones. 



All specimens of this shell that I have yet seen have the valves displaced, the 

 left valve overlapping the right. De Verneuil and M'Coy both noted this fact, 

 and thought it to be a structural character. I feel convinced, however, that 

 the displacement is due to the absence of hinge-teeth to keep the valves in line 

 after they had been buried ; but doubtless there is some structural reason why 

 the left always overrides the right valve. It is to be noted that the displacement 

 in M'Coy's specimen is practically nil, and that it gapes slightly at the anterior, 

 but not at the posterior, end; but the absence of gaping in this position is 

 possibly due to the slight displacement of the valves at this portion of the shell. 



Genus Clinofistha, Meek and Worthen, 1870. 



SoLEMYA (pars), dc Ryckholt, 1853. Melanges piilcontol., pt. 2, ]). 53. 

 Ebmondfa (pars), Hall, 1858. Geol. Kep. Iowa, vol. i, pt. 2, p. 71G, pi. xxix, 



fig. 3. 

 Clinopistua, Meek and Worthen, 1870. Proc. Acad. Nat. Sci. Philadelphia, 



p. -13. 

 — Meek, 1871. Proc. Acad. Nat. Sci. Philadelphia, p. G7. 



Tellinomya (pars), ILdl and Whifjiehl, 1872. Twenty-fourth Ami. Eeport N. 



York Mus. Nat. Hist., p. 192. 

 Clinopistha, Meek and Worthen, 1873. Geol. Surv. Illinois, Pal., vol. v, p. 581, 



pi. xxvii, fig. 7. 

 Dtstactella, Hall, 1883. Pal. New York, vol. v, pt. 1, p. 513. 

 Clinopistha, White, 1884. Eeport Geol. Surv. Indiana, 1883, p. 117, pi. xxxi, 



figs. 6, 7. 



