ART. 23 FRESH-WATER MUSSELS^ AKODOISTTITES MARSHALL. 3 



Besides these two shells of which Linnaeus has spoken, and which he has 

 designated in his works under the names of Mytilus cyf/neus (Anodonta 

 cygnca L.) and Mytilus anatinus {Anodonta anatina L.), I know seven other 

 species, not counting the one of which I shall speak, most of which have not 

 yet been described. 



The genus Anodontites takes its name from the hinge of the shells that 

 inclose it, which is plain and without teeth, and consequently very different 

 from the hinge of the other regular bivalve shells. It differs from the 

 shell of the Movie (Mussel. Mytilus), not only by this circumstance, which is 

 applicable to only a few of the species of this latter genus, but also by the form 

 of the shell, which is higher than long in the Moule (Mussel. Mytilus), and 

 fixed by a byssus, while the shell of Anodontites is longer than high and always 

 free. Above all they dilfer by the numi)er of muscle scars, which never exceed 

 two in the Motile (Mussel. Mytilus), but always number three in Anodontites, 

 without counting some light scars in the cavity of the beaks, which may 

 furnish new attachments to the animal of some species, but which are not 

 visible in others. 



This last character merits considerr.tion. It is common with the genus 

 Mulete (Unio) but is never found in any marine shell, and may thus seive to 

 prove the fluviatile origin of the shells in which it occurs. One should not, 

 however, conclude that all fluviatile bivalves have three muscle scars, for 

 there is also a genus undescribed and equally distinct from the genera Mulete 

 (Unio) and Anodontites, of which the shells have only two scars and which 

 live in fresh water only. 



If the best method of conchology should be that which bears equally upon 

 animal and shell, one may conclude that until the knowledge of the worms * is 

 sufiiciently advanced to undertake this vvork with success, it is at least neces- 

 sary to consider in shells those parts that offer the most relations with the 

 structure of the animals, or at least with some notable part of their organiza- 

 tion, among which there is no doubt that the muscle scars deserve first rank. 

 If Linnaeus had had regard for this essential part of the shell he would not 

 have introduced in the genera of the Moule (MusseL Mytilus), Huitre (Oyster. 

 Ostrea) and Hyrondes {Avicula, etc.), which have only one attachment in each 

 valve, the Cardita, which has two, and Anodontites, which has three. Further- 

 more, he would have distinguished the Anomias from the Terebratuias by this 

 single character, independent of those furnished by the regularity or irregular- 

 ity of the shell and by the hinge. 



Anodontite 



Characters of the genus : 



Shell, bivalve, longitudinal, regular, 

 free. 



Valines, equal, inequilateral, closing 

 throughout, nacreous within. 



Mtiscle scars, three in each valve ; one 

 near the posterior border, two un- 

 equal, united or distant near the 

 anterior border. 



Beaks, always eroded. 



Hinge, edentulous, not grooved. 



Ligament, exterior, slightly convex. 



Anodontites 



Charact. generis: 



Testa, Bivalvis, transversa, regularis, 



libera. 

 Valvulae, aeqitales, inaeqnilaterae, un- 



dique elausae, intvs margnritaceae. 

 Impressiones niusculares, tres in un- 



aquaque valvula; una prope mar- 



ginem anterioroii, duae inacqucles 



unitae nut distantes juxta marginem 



posleriorem. 

 Apices, semper erosi. 

 Cardo, edentulus nee canaliculatvs. 

 Ligamontum, extcrius parum promi- 



nens. 



* MoUuaks formerly were classed with woims. — w. I3. M. 



