282 FOSSIL OSTKEID^ OF NORTH AMERICA. 



known as the Monomyaria, or those whose shells are closed and held 

 together by only one adductor muscle, [a) 



Other bivalves, such as the common clam, for example, are known as 

 the Dimyaria, and their two shells are closed and held together by two 

 adductor muscles. As a rule, the shells of the Dimyaria are symme- 

 trical, and the individuals of the various species are constant and regular 

 in their respective shapes and in their ornamentation. It is also to an 

 almost equal degree the rule that the shells of the Monomyaria are 

 asymmetrical, and many of them also exhibit wide individual variation. 

 There are, however, noted exceptions to both these rules ; such, for ex- 

 ample, as those which are presented by the unsymmetrical genera Ghama 

 and Mulleria (b) among the Dimyaria, and by the symmetrical Pinna 

 and some species uf Pevten among the Monomyaria. 



Among the shells that are familiarly known, those which are most 

 nearly related to the Ostreidie belong to the genera Anomia, Pecten, 

 Spondylus, Avieula, and Pinna. The most unsymmetrical of these be- 

 long to the genera Anomia and Spondylus, but the Ostreidae exceed all 

 other molluscan shells in asymmetry and extravagant variation. 



Different authors have, from time to time, proposed separate generic 

 names for certain more or less distinct groups of forms amon g the Ostreidae, 

 the greater part of which I regard as not even of subgeneric value. Some 

 authors, on the other hand, I'eject all generic distinctions of the family 

 Ostreidae aS it is presented in this memoir, and refer all the species to 

 one and the same genus, the genus Ostrea of Linnaeus. While it is no 

 doubt true that among the fossil Ostreidae, intermediate forms may be 

 selected, which will closely unite all the groups of forms for which sepa- 

 rate generic names have been proposed, it is regarded as expedient, if 

 not actually necessary, to recognize among them at least two genera 

 besides the genus Ostrea proper, and also one subgenus of the latter. 

 These two genera and the subgenus referred to are recognizable only 

 among the fossil forms, and only the genus Ostrea proper is recog- 

 nized among the Ostreidae now living in North American waters. Fur- 

 thermore, although a considerable number of species of Ostrea have 



ain Mr. Ryder's article, on following pages, this muscle is described, and is also illus- 

 trated In the figures on Plate LXXIII. Its place of atiacliment to the shell is a con- 

 spicuous feature of the inner surface of each valve, as may he seen hy referring to those 

 figures on the accompanying plates which represent the inner surface of the shells. 



6The genus MUUeria is a strangely modified group of the Uaionidio, or fresh-water 

 clams. It is the shells of this genus, together with those of the related genus Aetheria 

 (the former living in South American, .and the latter in African rivers), that have been 

 called fresh-water oysters. In their habits of growth and in their forms the shells of 

 these two genera are suprisingly like those of true ojsters, but they plainly do not 

 belong to the OstroidiB. Mulleria is said to possess no trace of the anterior adductor 

 muscle in the adult and fixed st.ate, in which respect it still further simulates the 

 true oysters. But both of the adductor muscles are present in the young state and 

 the shells are then also equivalve and symmetrical, like those of ordinary Uniones. 

 Like them, the shell of Mulleria is also free in the young state. 



