20 NON-MARINE FOSSIL MOLLUSCA. 



passed over here without reference to the fact that Dr. S. G. Morton, 

 Mr. T. A. Conrad, and Dr. Isaac Lea have all described fossil species 

 which they regarded as belonging to the Unionida;. They were, however, 

 either incorrectly referred to that family, or the formations from which 

 they were respectively obtained are incorrectly stated; and they are, 

 therefore, enumerated under the head of spurious and doubtful species 

 on following pages. 



That the Unionida; existecl within the area that now constitutes West- 

 ern North America in the Jurassic period, and that the genus Unio of 

 Eetzius bad then not only become established, but had reached a good 

 degree of differentiation as regards the establishment of subordinate 

 groups of forms within that great genus, is apparently beyond reason- 

 able doubt; and in this article the question is treated as affirmatively 

 settled. At the same time it should be stated that in the case of at 

 least a majority of the alleged discoveries of fresh-water inolluscan 

 species in Jurassic strata, some doubt has been thrown upon the gen- 

 uineness of the fresh- water origin of the strata in which they were de- 

 posited, or upon the actual Jurassic age of those strata. 



The first discovery in North American Jurassic strata of shells which 

 are referable to the Unionidse was announced by Meek & Hayden in 

 connection with the publication of Unio nucalis,* which is figured on 

 Plate 3. Those authors, however, expressed a remote doubt as to 

 whether the strata in question, which occur in the vicinity of the Black 

 Hills, are really of Jurassic age. No other examples of this species be- 

 sides the type specimens have ever been discovered. They are shown to 

 be those of true Unio by the hinge characters observable upon one of the 

 specimens; and the outer prismatic shell-layer is observable on all of 

 them. They were found associated with shells, which Meek & Hayden 

 referred to the genera Planorbis, Valvata, Yiviparus, Neritella,, and 

 Lioplacbdes respectively. 



Another Jurassic species referable to this famdy is Unio stewardi, 

 White, which was described from some imperfect si>eciinens that were 

 collected from Jurassic strata by Mr. J. F. Steward in Northern Utah.t 



Fig. 1, on Plate 3, is an outline illustration of this species which 

 has been made up by help of several fragments, no perfect example 

 having ever been discovered. It is believed to represent closely the 

 outline and general aspect which the species presented while living. It 

 is an interesting form, because it illustrates the fact that at least one 

 of the subordinate types of Unio that now exists among the living 

 species of the Mississippi Eiver system was established at that early 

 epoch. 



The next known member of the Unionidse, the appearance of which is 

 to be mentioned in the order of geological time, is an interesting form 



* Paleontology of the Upper Missouri, p. 92, pi. iii, fig. 13. 

 t Powell's Report, Geology of the Uinta Mountains, p. 110. 



