302 PALEONTOLOGY OF NEW YORK. 



encrusted. Possibly a similar occurrence explains the apparent union of the 

 crural apophyses described by White and St. John as occuring in Cryptacan- 

 THiA. The external form of the Burlington limestone species is apparently 

 more biconvex than in Cryptacanthia compada, and the generic characters of 

 both are extremely uncertain. 



Genus T ROP IDOL E PTU S, Hall. 1857. 



PLATE LXXXn. 



1889. StTophmnena, Cohrad. Ann. RepU Oeolog. Dept. N. Y., p. 64. 



1847. Leptana, dk Vrenbdil. BuU. 8oc. G£oL de Prance, second ser., voL iv, p. 706, pi. iii, 



figs. 7, 7o. 

 1853. Leptana, Schndr. Palteontographica, vol. iii, p. 220, pi. xl, fig. 2. 

 1856. Strophomena, Thb SANDBERaERS. Verstein. der rhein. Schichten-systems, p. 66, pi. xxxiv, 



fig. 8. 

 1867. Tropidoleptut, Hall. Tenth Ann. Kept. N. Y. State Cab. Nat. Hist., p. 151, figs. 1, 2. 

 1859. Tropidoleptiis, Hall. Twelfth Ann. Kept. N. Y. State Cal.. Nat. Hist., p. 31. 



1866. Leptaena f Uavidsok. British Devonian Brachiopoda, p. 87, pi. xvii, figs. 1-3. 



1867. lyopidoleptus, Hall. Palteontology of New Yoi-k, vol. iv, p. 404, pi. ixi a, figs. 60-62 ; pi. Ixii, 



figs. 2o-c, 3a-y. 



1868. Tropidoleptux, Meek and Worthem. Geology of Illinois, vol. iii, p. 427, pi. xiii, fig. 2. 

 1874. Tropidoleptus, Rathbdk. Bull. Buffalo Soc. Nat. Sci., vol. i, p. 254, pi. ix, fig. 10. 

 1876. Tropidoleptus, Derby. Bull. Mus. Harvard Coll., vol. iii. No. 12, p. 282. 



1881. TVopidoleptus, Rathbck. Proc. Boston Soc. Nat. Hist., vol. xx, p. 35. 



1889. TwpidoUptus, Nbttelhoth. Kentucky Fossil Shells, p. 46, pi. xvii, figs. 14, 15. 



1890. DropidolepUis, Derby. Arch. Mus. Nac. de Rio de Janeii'o, vol. ix, p. 76. 



1892. Tropidoleptus, Ulrich. Paleoz. Verstein. aus Bolivien ; Neues Jahrb. ftlr Minei-al., etc., Bei- 

 lagebnd. viii, p. 73, pi. iv, figs. 32-34. 



Diagnosis. Shells with the general external aspect of Rafinesquina ; 

 concavo-, or plano-convex. Hinge-line straight; in young shells form- 

 ing the greatest transverse diameter and frequently extended at the 

 cardinal extremities, but in mature and old shells shorter than the trans- 

 verse diameter in the pallial region. Marginal outline varying from 

 longitudinally semi-elliptical in youth, to transversely subelliptical at ma- 

 turity. Surface covered with simple, low plications, all extending from 

 beak to margins. The median plication on the pedicle-valve and the 

 corresponding sinus on the brachial valve are broader and more conspicuous 

 than the others. 



