WIIITE.1 INTRODUCTORY REMARKS. 283 



been proposed among the living Ostreidre of North America by different 

 authors, it is now a serious question whether there is really more than 

 one species of oyster now living in the Atlantic waters of North America. 

 Those of the west coast of North America are not so well known, but 

 at present three or four species are recognized there by the different 

 authors who have studied them. 



The shells of the Ostreidfe are ineqnivalve; that is, the two valves 

 which, like those of all other true bivalve mollusks, are right and left 

 respectively, are unequal in shape. The moUusk in a very early stage 

 of its existence becomes attached to a foreign object, and the left valve 

 also becomes attached to that object, leaving the right valve free. Some- 

 times the left valve becomes attached to the foreign object by the 

 greater part of its exterior surface ; but sometimes only by the point of 

 the beak. When the surface of attachment is large, the shell to a great 

 extent conforms to the surface upon which it is attached ; and such 

 shells are necessarily irregular in shape. When the surface of attach- 

 ment is small, the shell is more regular in shape, but in no case are the 

 valves wholly regular. 



The three genera that are recognized in this memoir, and which have 

 already been referred to, are JExogyra, Gryphma, and Ostrea, and the 

 subgenus of the latter is Alectryonia. (a) Exogyra costata, Say, may be 

 taken as the type of this genus, and E.forniculata, White, as an interme- 

 diate form between Exogyra and Gryplicea. The former is represented on 

 Plates LVI and LVII, and the latter on Plate LII. The shells of 

 Exogyra are usually more regular in shape than those of any other 

 genus of the Ostreidre, but, as already stated, they are never wholly 

 regular, and they are often much distorted. The left or under valve is 

 always much the deeper, and the right or upper valve is usually flat. 

 The beak of the lower valve is always turned strongly to the rear, (&) and 

 it is usually more or less spiral. The inconspicuous beak of the upper 

 valve also partakes of this deflected or spiral character. The shells of 

 Exogyra are also, as a rule, more massive than those of any other of 

 the Ostreidfe. This is especially true of the lower valve, which in 

 E. costata and E. ponder osa sometimes reaches an inch and a half in 

 thickness of solid shell substance. The genus Exogyra was a widely 

 differentiated one in the Cretaceous period, within the region that now 

 constitutes North America. It was not only represented by a consider- 

 able number of species, but those species embraced a wide variation of 

 form, as may be seen by reference to the figures that represent them on 

 the accompanying plates. 



The genus Gryplum is not so well represented in North American strata 



a The species which Conrad proposed to group under the generic name of Gryphw- 

 oslrea I regard as not being even subgenerically distinct from Ostrea proper. See re- 

 marks underthe head of Ostrea vomer on a following page. 



t That is, to the left hand as one looks down upon the cavity of the valve, the hinge 

 border being away from the person. 



