LAKE LARAMIE 201 



Europe. The following is a fairly complete list : 

 Unionida : Unio ; Cyrenidcz : Cyrena, Corbula ; Valvatidce : 

 Valvata ; Hydrobiidce : Hydrobia, Ainnicola (now living 

 in North America), Bithynia ; Neritidce : Neritina ; 

 MelaniidcB : Pleurocera, Lioplax, Goniobasis, Leptoxis, 

 Ptychostylus ; Auriculidce : Auricula, Carychium ; Lim- 

 ncBidcB : Limnsea, Pianorbis ; Mactridce : Gnathodon. 

 The last-named is commonly classed as a sub-genus 

 of Mactra ; it may have become specially modified in 

 middle Jurassic times, for the genus is not known earlier 

 than the Coral rag. Gnathodon still inhabits fresh water 

 in the region of the Gulf of Mexico, and the Melanids, 

 which occur in rich development, are closely related to 

 North American forms. Beyond this the list calls for no 

 special comment ; many of the more important genera 

 were, as we have seen, in existence at an earlier period. 



Now we ascend a step higher in the series, and enter 

 the uppermost part of the Cretaceous system. In North 

 America this is represented by the Laramie beds, the 

 sediments of a great lake or inland sea, which covered an 

 area of 50,000 square miles. In the north the Laramie 

 beds extend through Montana and Dakota into British 

 Columbia ; in the south they reach New Mexico ; towards 

 the west they may be traced as far as the south-west of 

 Utah ; they disappear east of the Kocky mountains, but 

 probably underlie a part of the Tertiary beds of the great 

 central plain. Their thickness is at least 5,000 feet, 

 and they pass gradually down into the marine Cretaceous 

 sediments below them. An inland sea of such magnitude 

 would be long before it became completely freshened by 

 the influx of rivers, and the passage from salt water to 

 brackish or freshwater conditions would be accomplished 

 in the most gradual manner. We are thus presented 

 with all the circumstances favourable to the success of 

 a grand experiment in the transformation of species. If, 



