BIG BADLANDS OF SOUTH DAKOTA. 



73 



/06 03,X%.. 



12705, X 3/4 



Fig. 3. A, maximum and B, minimum size variants of Hyracodon apertus 

 sp. nov., of which No. 10803 is the type. Upper premolar-molar series of the 

 left side in crown view, three fourths the natural size. No. 10803, 12705 

 Princeton University Geological Museum. 



For the species described as C v^e are without a name and I pro- 

 pose that it be known as Hyracodon apertus sp. nov., with reference 

 to the wide-open valley of p* as described under C above, designating 

 as type a skull with right ramus of the lower jaw, No. 10803, Prince- 

 ton University Geological Museum, collected by Mr. J. B. Hatcher in 

 1893 from a zone of brown nodules above the Protoceras sandstones 

 in Corral Draw, South Dakota. A crown view of the premolar- 

 molar series is given in Fig. 3 A. Regrettable as the proposing of 

 additional specific names may be, it is unavoidable in this instance in 

 order to designate a form already adequately figured by Leidy in 

 Fig. 13 on Plate XIV., "Ancient Fauna," but grouped by him with 

 nebrascensis, from which, however, the absence of blocking in the 

 valley of p* easily separates it.^^ To this species belongs the mounted 



13 Long-continued wear would, undoubtedly, isolate a central depression 

 as shown in Leidy's figure of nebrascensis ("Ancient Fauna," PI. XV., Fig. 

 3) or in Fig. 2 C. At corresponding stages of wear, however, p^ is unblocked 

 in apertus and completely blocked in nebrascensis (compare Figs. 2A and 3^). 

 The development of a transverse spur from the protoloph would convert p * 

 of apertus into a tooth of the nebrascensis type ; its loss in nebrascensis would 

 result in the structure found in apertus. 



