310 A7i Anodonta livhig in perpendicular holes. 



it is chiefly in the order Pulmo-branchia?, in the class of Cepha- 

 lidii, and in the family Submytillaceae, and likewise in the class 

 of Acephalidii, that M. d'Orbigny has collected shells, and made 

 observations and drawings from the living subject, highly de- 

 serving of attention. We now know that the genera Unio and 

 Anodonta, both of them so rich in varied and remarkably formed 

 shells in the lakes and rivers of North America, are likewise very 

 numerous in the tributaries of La Plata, which flow down the 

 eastern side of the Cordilleras in Paraguay. Among the most 

 interesting, we have remarked a species of Anodonta, the shell 

 of which is formed like a lithodome muscle, or of a pholas, and 

 which lives placed perpendicularly in a hole, rising and descend- 

 ing by a peculiar kind of mechanism in the structure of the foot ; 

 several true unios, provided with a respiratory tube still more 

 developed than in the iridinia of the Nile ; lastly, certain species 

 in which the hinge demonstrates, better than any specimen pre- 

 viously in conchyliological collections, the passage of the genus 

 Castalia of Lamarck to the unios. 



When crossing the ocean, M. d'Orbigny neglected no oppor- 

 tunity that occurred to him of examining the biphores, diphyes, 

 and heroes. While studying these, drawing and colouring 

 them from living examples, he doubtless will have observed 

 some new facts, or his observations will serve to complete what 

 was imperfectly known ; and it is more than probable that he 

 has found some new species which had escaped the researches of 

 MM. de Chamisso, Eschscholtz, Quoy, and Gaymard, to whom 

 the science owes so many interesting facts. 



The same observations may be made respecting the medusae 

 or types of the radiated animals, which, as being pelagic, were 

 necessarily more frequently presented to M. d''Orbigny''s obser- 

 vations, than the echinidae, madrepores, polypiariae, and zoophy- 

 tariae, of which he has brought home but a small number, as 

 these animals are more or less littoral, being usually found fixed 

 to rocks. Indeed we have noticed a considerable number of 

 medusas belonging to the genera Equorea, Geronya, Aurelia, 

 Chrysaoris, and Rhysostoma, drawn and coloured with sufficient 

 care to admit of being employed, in the absence of the animals 

 themselves, which are so difficult to preserve, in illustrating a 

 work which may benefit the science. 



