200 



FOR M A T I O N S OF THE 



Naturalists who have inspected the fossil turtles from the Mauvaises Terres, have 

 found great difficulty in deciding whether they should be referred to the genus 

 Emys or to Testudo. 



It is, I believe, agreed that, in their general aspect and contour, they resemble 

 more the usual form of aquatic turtles than the terrestrial tortoise ; but Dr. Leidy 

 has shown, from the details of their comparative anatomy, that in certain structures 

 of the intercostal portion of their shelly covering, and in the intervention of a 

 peculiar and somewhat rhomboidal-shaped plate, in the posterior part of the cara- 

 pace, they approximate more closely to the Testudo, and differ, in this respect, from 

 all living Emys, he has had an opportunity of examining ; and, for this reason, 

 he has referred these fossil turtles, brought from the Mauvaises Terres, to the 

 former genus. Nevertheless, it is highly probable, from the great number of their 

 remains discovered in a limited space, that they were aquatic in their habits, and, 

 if so, may form an intermediate genus or subgenus — a lost link — between the exist- 

 ing aquatic turtles and terrestrial tortoises. For further particulars on this head, 

 I must refer the reader to Dr. Leidy's interesting memoir. 



These turtles were chiefly observed in a portion of the " Bad Lands," some five 

 or six miles in extent, which has much the appearance of an ancient lake, where 

 it is entered from Bear Creek, a tributary of the Cheyenne.* At one of these lake- 

 like expansions, hundreds of fossil turtles were discovered. They do not rest 

 immediately on the grassy plain that forms the present floor or bottom, but on the 

 talus and debris, collected into mounds, which have been derived from the disintegra- 

 tion of the marly earths that have slid from above. The particular stratum in which 

 they seem to have been originally embedded, is a pale flesh-coloured, indurated, 

 siliceous, marly limestone, situated from thirty to fifty feet above, as shown in No. 

 7, of the following section. 



SECTION OF BEDS CONSTITUTING THE EARLY TERTIARY (EOCENE) OP THE BAD LANDS 



(MAUVAISES TERRES). 



(Numbered in the descending order.) 



Feet. Inches. 



1. Ash-coloured clay, cracking in the sun, containing siliceous concretions, 30 



2. C mract white limestone, ...... 3 



3. Light-gray marly limestone, ...... 8 



4. Light-gray indurated siliceous clay (not effervescent), . . .30 



5. Aggregate of small angular grains of quartz, or conglomerate, cemented 



by calcareous earth, slightly effervescent, .... 8 



6. Layer of quartz and chalcedony (probably only partial), . . 1 



7. Light-gray indurated siliceous clay, similar to No. 4, but more calcareous, 



passing downwards into pale flesh-coloured, indurated, siliceous, 

 marly, limestone (effervescent), turtle and bone bed, . . 25 



8. White and light-gray calcareous grit, slightly effervescent, . 15 



9. Similar aggregate to No. 5, but coarser, .... 8 

 10. Light-green, indurated, argillaceous stratum ; (slightly effervescent) ; 



Palseotherian bed, ....... 20 



* See small map. 



