100 CLASS I. TROPIOPODA. ORDER I. BIMUSCULOSA. 



PL LXXVI. Fig. 19. 

 Cardium tenuicostatum, Lamarck, Anim. sans vert., new edit., vol. vi. 

 p. 392. 



PL LXXVI. Fig. 20. 



Cardium paucicostatum, Sowerby, Jun., Proceedings Zool. Soc, 1840, 

 p. 106. 



PL LXXVI. Fig. 21. 



Cardium Panamense, Sowerby, Proceedings Zool. Soc, 1833, p. 85. 



PL LXXVII. Fig. 22. 



Cardium tumoriferum, Lamarck, Anim. sans vert., new edit., vol. vi. 

 p. 410. 



PL LXXVII. Fig. 23. 



Cardium procerum, Sowerby, Proceedings Zool. Soc, 1833, p. 83. 



PL LXXVII. Fig. 24. 



Cardium subretusum, Sowerby, Jun., Proceedings Zool. Soc, 1840, 

 p. 110. 



PL LXXVII. Fig. 25. 

 Cardium planicostatum, Sowerby, Proceedings Zool. Soc, 1833, p. 83. 



ISOCARDIA, Lamarck. 



Testa aequivalvis, cordata, ventricosa ; umbonibus distantibus, divarica- 

 tis, involutis. Dentes cardinales duo compressi, uno sub umbone 

 recurvo ; dente laterali antico, elongato. Impressio muscularis 

 pallii simplex. Ligamentum externum. 



The cordate symmetry of form which distinguishes the shell of Iso- 

 cardia, as well as the elegant incurvature of the umbones, render it emi- 



