1S69.] 261 (Cope. 



Mitchillii and M. Camperi, are among tlie most elongate of animals. 

 Tkej are only exceeded by some of the whales of the present day. 

 Add to this their slender proportions, with no doubt, powers of 

 swimming in the ocean, running, springing and climbing on land, and 

 we have a combination of characters more fonnidable than those of the 

 cimoliasaurs, elasmosaurs and crocodiles of that age of great reptiles. 

 Leidy observes that the varieties of form in the teeth indicate un- 

 usual variation for a single species, or else a larger number of species 

 than has been hitherto supposed. I adopt the latter view after a 

 comparison of extended material, as I find the most marked peculiar- 

 ities in the quadrate bones and vertebras, in addition to those of the 

 teeth. 



I. The posterior dorsals elevated, and with subpentagonal section. 

 Mandibular teeth fourteen on each ramus; premaxillaries six; 



pterygoids eight, of moderate size j\I. f/iganteus. 



Mandibular teeth twelve, spaced. Size smaller . . 31. gracilis. 



Mandibular teeth ? ; premaxillaries four, pterygoids eight, sub- 

 equal ; the shaft of the humerus slender sub-cylindric; squamo- 

 6al bone without horizontal expansion on the ojiisthotic ; quadrate 

 bone longer than broad, its proximal extremity an open sigmoid 

 with a vei-y small continuation on the edge of the ala; teeth more 

 or less facetted il/. MitdiiUii. 



Quadrate bone above a closer sigmoid, with a long, wide continua- 

 tion on the edge of the ala ; knob inside the meatus very smalL 

 M. maxbnus. 



Teeth compressed, without facets, the pterygoid unequal, the me- 

 dian veiy large; together eight M. impar. 



II. The posterior lumbars with depressed centra, and ovate ex- 

 tremities- 



u. Large species. 



i*. Anterior lumbars little depressed- 



Mandibular teeth fourteen; pterj'goids ten; squamosal with broad, 



triangular expansion above opisthotic. Quadrate bone longer than 



broad. Dorsals transversely ovate, sides rounded . M. missuriensis. 



^ (i. Anterior lumbars flattefted like the posterior. 



Dorsals flattened, with lateral keel on side; caudals vertical and 



ovate 31. Brumbiji. 



a a. Small species. 

 Centra transversely ovate; caudals vertical ovate . . 31. minor. 



