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 TROPIDOLEPTUS, Hall. 1857, 



PLATE LXXXU. 



1839. Strophomena, Conrad. Ann. Kept. Geolog. Dept. N. Y., p. 64. 



1847. Leptcena, dk Vebnbdil. BulL Soc. Geol. de France, second sei\, vol. iv, p. 705, pi. iii, 



figs. 7, 7 a. 

 1853. LejjicBna, Schnur. PalsBontog-i-aphica, vol. iii, p. 220, pL xl, tig. 2. 



1856. Strophomena, The Sandbergbrs. Verstein. der ihein. Schichten-systems, p. 66, pL xxxiv, 



fiff. 8. 



1857. Tropidoleptus, Hall. Tenth Ann. Kept. N. Y. State Cab. Nat. Hist., p. 151, figs. 1, 2. 

 1859. Tropidoleptus, Hall. Twelfth Ann. Kept. N. Y. State CaL. Nat. Hist., p. 31. 



1865. Leptana? Davidson. British Devonian Brachiopoda, p. 87, pi. xvii, figs. 1-3. 



1867. Tropidoleptus, Hall. Palaeontology of New York, vol. iv, p. 404, pi. ixi a, tigs. 50-52 ; \<\. Ixii, 



figs. 2a-c, Za-y. 



1868. Tropidoleptus, Meek and Worthbn. Geology of Illinois, vol. iii, p. 427, pl. xiii, fig. 2. 

 1874. Tropidoleptus, Rathbon. Bull. Bufl'alo Soc. Nat. Sci., voL i, p. 254, pL ix, fig. 10. 

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



1881. Tropidoleptus, Rathbdn. Proc. Boston Soc. Nat. Hist., vol. xx, p. 35. 



1889. Tropidoleptus, Nettelroth. Kentucky Fossil Shells, p. 46, pl. xvii, figs. 14, 15. 



1890. Tropidoleptus, Derby. Arch. Mus. Nac. de Rio de Janeiro, vol. ix, p. 76. 



1892. Tropidoleptus, Ulrich. Paleoz. Verstein. aus Bolivien ; Neues Jahrb. filr Mineral., etc., Bei- 

 lagebnd. viii, p. Ti, pl. iv, tifrs. 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 vfilve are broader and more conspicuous 

 than the others. 



