I02 AMERICAN MESOZOIC MAMMALIA 



The status of this generic name, as here used, was sharply challenged by Marsh 

 who pointed out that among the original specimens of Cope was a dinosaur tooth which 

 was included in the definition. Osborn, however, accepted the name on grounds which 

 seem to be valid and it is the one now in common use. It is true that the genus was 

 based on three specimens, a mammalian M^ an ankylosaurid dinosaur tooth, and the 

 distal end of a mammalian humerus, but the mammal tooth was clearly considered as 

 the type and the name should be taken as applying to it alone. The humerus fragment 

 probably does belong to Menisco'essus also. 



Meniscoessus, sensti lata, shows relatively little variation in homologous parts of 

 the dentition and includes either a single natural genus or at most two very closely 

 related genera. The original specimen seems to be unique in some respects and the 

 name Meniscoessus conquistus is limited to it. The Montana specimens probably be- 

 long to a single species and differ slightly from the South Dakota or Wyoming mate- 

 rial. They may hence be included in a distinct species which has been called Menis- 

 coessus borealis. The Wyoming specimens probably include several species, but these 

 cannot be usefully separated at present and all are included in M. robustus, sens, lat., 

 at present, using the first trivial name applied to a specimen from this region. 



Meniscoessus conquistus Cope 1882 

 1882. M. conquistus, Cope, Am. Nat., XVI, 830. 



Type. — Amer. Mus. No. 301 1. M*, broken. Fig'd, Cope 1884, fig. 7. 



Horizon and Locality. — "Laramie" (i.e.. Lance equivalent). South Dakota. 



Diagnosis. — Outer cusp row of M^ with at least four distinct cusps and nearly as 

 long as the median row. First cusp of inner row not well separated from second. Width 

 5.8 mm., estimated length about 6.6 mm. 



Meniscoessus borealis Simpson 1927 

 1927. M. borealis, Simpson, Amer. Mus. Novitates, No. 267, 3. 



Type. — Amer. Mus. No. 14411. Isolated M^ left. 



Horizon and Locality. — Hell Creek beds, near head of Crooked Creek, Dawson 

 County, Montana. 



Diagnosis. — Cusp formula of M^ 4:4:5, first cusp of inner row more distinct 

 from second than in M. conquistus, measuring 8.3 mm. in length by 7.0 mm. in width. 



In addition to the type, M\ V, Ii, and P4 are known at least in part. They agree 

 in general with those of M. robustus, below. M^ is considerably larger than in M. con- 

 quistus and is slightly larger and distinctly more elongate than in M. robustus. 



Meniscoessus robustus (Marsh 1889) 

 1889. Di-priodon robustus. Marsh, Amer. Jour. Sci. (3) XXXVIII, 84. 



Type.— Y.P.M. No. 11234. Isolated M2 right. Fig'd, Marsh 1889A, PI. II, figs. 

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