302 
PALAEONTOLOGY OF NEW YORK. 
encrusted. Possibly a similar occurrence explains the apparent union of the 
crural apophyses described by White and S r. John as occuring in Cryptacan- 
THiA. The external form of the Burlington limestone species is apparently 
more biconvex than in Cryptacanthia compacta, and the generic characters of 
both are extremely uncertain. 
Genus TROPIDOLEPTUS, Hall. 1857. 
PLATE LXXXU. 
1831). Strophomena, Conrad. Ann. Kept. Geolog-. Dept. N. Y., p. 64. 
1847. Leptasna, de Vbhnbdil. Bull. Soc. Geol. de France, second ser., vol. iv, p. 705, jil. iii, 
figs. 7, 7 a. 
1853. LeptCBna, Schnur. Palseontograpbica, vol. iii, p. 220, pi. xl, fig. 2. 
1856. Strophoinena, Thb Sandbergers. Vei-stein. der rhein. Schichten-systems, p. 66, pi. xxxiv, 
fig. 8. 
1857. Tropiddleptus, Hall. Tenth Ann. Kept. N. Y. State Cab. Nat. Hist., p. 151, figs. 1, 2. 
1859. TropidoJeptus, Hall. Twelfth Ann. Kept. N. Y. Stale Cab. Nat. Hist., p. 31. 
1865. Leptaina ? Davidson. British Devonian Brachiopoda, p. 87, pi. xvii, figs. 1-3. 
1867. Tropidoleptus, Hall. Palaeontology of New York, vol. iv, p. 404, pi. IxiA, figs. 50-52; pi. Ixii, 
figs. 2a-e, 3a-y. 
1868. Tropidoleptus, Meek and Worthen. Geology of Illinois, vol. iii, p. 427, pi. xiii, fig. 2. 
1874. Tropidoleptus, Rathbdn. Bull. Buffalo Soc. Nat. Sci., vol. i, xi- 254, pi. ix, fig. 10. 
1876. Tropidoleptus, Derby. Bull. Mus. Harvard Coll., vol. iii. No. 12, p. 282. 
1881. Tropidoleptus, Rathbun. Proc. Boston Soc. Nat. Hist., vol. xx, p. 35. 
1889. Tropidoleptus, Nettelroth. Kentucky Fossil Shells, x^- 46, pi. xvii, figs. 14, 15. 
1890. Tropidoleptus, Derby. Arch. Mus. Nac. de Rio de Janeiro, vol. ix, x’- 76. 
1892. Tropidoleptus, Ulrich. Paleoz. Verstein. aus Bolivien ; Neues Jahrb. fiir Mineral., etc., Bei- 
lagebnd. viii, xi- 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. 
