white.] ANNOTATED CATALOGUE. 21 



which was described by Meek & Hayden,* from Southeastern Dakota, 

 under the name Margaritana nebrascensis. It was obtained from the Da- 

 kota Group, which is the earliest group of the Cretaceous strata in the 

 North American series ; and it was found associated with (Jyrena dakotensis 

 and Pharella f dakotensis of the same authors. These forms indicate a 

 brackish- water, probably estuary, origin for at least the layers in which 

 those fossils occur, although the remains of marine mollusca are found 

 in other parts of the same group. Margaritana nebrascensis is repre- 

 sented by two figures on Plate 4, which figures are drawn from the prin- 

 cipal type specimen. 



In shape and general aspect this shell resembles some of the living- 

 forms of Margaritana, but it is of a somewhat different type from any 

 known living species of that genus. It differs still more from any of the 

 other known fossil Omonidae, except the form which was described by 

 Gabb t from the Cretaceous strata of Vancouver's Island under the name 

 of U. hubbardi. This suggestion of congeneric relationship is based 

 upon the external characteristics alone which both present, because 

 nothing is yet known of the character of the hinge of U. hubbardi. The 

 hinge of M. nebrascensis, as ascertained by Meek, seems to warrant its 

 reference to the genus Margaritana, and if this reference is correct, that 

 species is, with the probable exception of the U. hubbardi of Gabb, the 

 only known North American fossil form which can be properly referred 

 to Margaritana. But, as before intimated, it probably belongs to a sub- 

 ordinate type of that genus which early became extinct. Unio hubbardi 

 is represented by copies of Mr. Gabb's figures on Plate 5. 



Mr. Gabb also described and figured f another species, a strange bi- 

 alate form from the Cretaceous strata of California, under the name of 

 Unio penultimus, of which he seems to have had very imperfect speci- 

 mens. Figure 1, on Plate 5, is a copy of Mr. Gabb's original figure 

 of this form. He expressed no doubt of the correctness of his refer- 

 ence of that species to the genus Unio, and, notwithstanding its unusual 

 form, there appears to be no reason to suppose that it may not belong at 

 least to the Unionidse. If it really is a member of this family it presents 

 an interesting example of a subordinate type which has become extinct, 

 while many co-ordinate types which existed contemporaneously with, or 

 shortly after it, have come down to the present time unchanged. It is 

 a significant and interesting fact, especially in connection with the refer- 

 ence of his Coalmeasure genus Naiadites to the Unionidse by Dawson, 

 that both of the Uniones which were described by Gabb from the Creta- 

 ceous strata of the Pacific coast were found in strata associated with the 

 coal beds of that period. 



The three last-mentioned species of the Unionidse are, with one excep- 

 tion, all that are at present known to have existed within the area which 



* TJ. S. Geol. Sur. Terr., vol. ix, p. 114. pi. i, fig. 5. 



t Paleontology of California, vol. ii, p. 190, pi. 30, fig. 86. 



t Paleontology of California, vol. i, p. 182, plate 24, fig. 164. 



