442 NON-MARINE FOSSIL MOLLUSCA. 



parts of Utah. It is represented on* Plate 8. Mr. Meek described 

 another form in connection with this under the name of C. englemannij 

 but the numerous examples that have been collected from those strata 

 show such a degree of variation as to make it very doubtful whether 

 the difference between the two forms is anything more than individual 

 variation. C. englemanni is therefore regarded as a synonym of C. 

 pyriformis. 



An interesting species, which occurs in the valleys of Bitter Creek, 

 Southern Wyoming, and Yampa River, Northwestern Colorado, was 

 described by Mr. Meek under the name of Corbula undifera, which 

 is illustrated on Plate 18. A .variety of this species, which also 

 occurs in the valley of Bitter Creek, but at a higher horizon in the 

 Laramie Group by several hundred feet, was described by me under 

 the name of C. subundifera.* Although the latter is now regarded as 

 only a variety of the former, yet the variation is quite constant and 

 readily recognizable, as may be seen by comparing the figures of the . 

 variety with those of the typical form on Plate 18. 



Two species were described by Meek & Hayden from the Judith 

 River beds of the Upper Missouri River region, under the names re- 

 spectively of Corbula subtrigonalis and C. perundata, both of which I 

 regard as belonging to one and the same species. Furthermore, the 

 two species which were described by Meek from the Laramie strata of 

 the valley of Bitter Creek, Southern Wyoming, under the names re- 

 spectively of G. tropidopJwra and C. crassatelliformis, I regard as also be- 

 longing to that species, making three synonyms of (7. subtrigonalis.] 

 Figures of these forms may be seen on Plate 19. 



One other published form of Corbula only remains to be noticed in 

 this article, namely, C. mactriformis^. which was described by Meek & 

 Hayden from the Fort Union beds of the Upper Missouri River region, 

 and which has hitherto been found nowhere else. It is represented 

 on Plate 18. 



The non-marine bivalve mollusca that have been noticed on the pre- 

 ceding pages comprise only six families, while, as we shall see, their as- 

 sociated gasteropod families were more than double that number. This 

 numerical proportion of the families pertaining to the Conchifera and 

 Gasteropoda, respectively, may be taken as about the same as that 

 which obtains in those two classes among living non-marine mollusca. 

 Indeed, the familieswhich have been recognized among these fossil forms 

 are no other than those under which we group their living represen- 

 tatives. 



*For descriptions and fignres of both these forms see An. Rep. U. S.-Geol. Sur. 

 Terr, for 1878, Part I, pp. 80, 81, pi. 29. 



t For descriptions and fignres of these forms and remarks upon them see U. S. Geol. 

 Sur. Terr., vol. ix, pp. 529, 530, pi. 40; An. Rep. U. S. Geol. Sur. Terr, for 1870, p. 

 315 ; ib. for 1872, p. 514 ; ib. for 1878, Part I, pp. 80, 81, pi. 25. 



t U. S. Geol. Sur. Terr., vol. ix, p. 528, pi. 42, fig. 7. 



