30 NOX-MARINE FOSSIL MOLLUSCA. 



bicula as to give an impression to the casual observer that the family 

 then reached a culmination, but it was in reality a culmination of the 

 genus Corbicula only. 



In the Cretaceous strata of the western part of the continent several 

 species belonging to the Cyrenida?. have been discovered, all of which 

 are referred to the genus Gyrena as distinguished from Corbicula.* Only 

 two of these species, however, are regarded as coining within the scope 

 of this article, because all the others are found to be associated with 

 such forms as are regarded as indicating a marine habitat, while those 

 two species are associated with non-marine forms. Although in the Cre- 

 tacious period the genus Gyrena was established with all its distinctive 

 characteristics, as they are at present known, and was represented by 

 several known species in the southern portion of North America, with 

 one or two exceptions the genus has not been recognized in strata of any 

 geological period since the Cretaceous. 



And again, although the genus Corbicula, formed so conspicuous a fea- 

 ture of the fauna of the Laramie period, it is at present not known to 

 occur in any North American strata of either earlier or later date than 

 that period; if we except the G. truncata of Prime, which is understood 

 to be of doubtful authenticity. These facts are significant as regards 

 the genetic history of the family, and show, among other things, that 

 our knowledge of that history is far from complete. 



The earlier of the two non-marine fossil species of Cyrena that have 

 just been referred to, is C. dakotensis Meek & Hayden,f which has al- 

 ready been mentioned on a previous page as an associate of Margari- 

 tana nebrascensis in strata of the Dakota Group in Southeastern Dakota. 

 It is represented on Plate 4 by copies of Mr. Meek's original figures. 



The other species is Cyrena carletoni Meek,t which is represented on 

 Plate 5, and which was discovered by Mr. Meek in the Cretaceous 

 estuary deposit at Coalville, Utah, which has already been mentioned in 

 connection with remarks upon Anomia propatoris. The only examples 

 of this species that have been discovered are small and delicate, and it 

 is probable that they are all young shells. 



Although the family Cyrenida? has evidently become well established 

 iu the estuary and marine waters of the Cretaceous period, at least by 

 its typical genus Cyrena, it is in the strata of the Laramie Group, as be- 

 fore stated, that we find evidence of its greatest development, especially 

 as regards the genus Corbicula. Among the fossil forms of that genus 

 which the Laramie Group has furnished, three sections or subgenera are 

 recognized, two of which have become extinct. 



Beginning with the Bear Biver beds of the Laramie Group, which are 

 perhaps somewhat earlier than the other known portions of the group, 



*For remarks on these genera, by Mr."Meek, see U. S. Geol. Sur. Terr., vol. ix, p. 

 157. 

 tU. S. Geol. S'ur. Terr., vol. ix, p. 159, pi. I, fig. 1. 

 t An. Eep. U. S. Geol. Sur. Terr, for 1878, Part I, p. 20, pi. 12, rig. 16. 



