X2 PROCEEDINGS OF THE NATIONAL MUSEUM vol.86 



RHINEURA MINUTUS, new species 



Figure 1 



Type. — U. S. N. M. no. 12158, consisting of the skull, lower jaws, 

 23 articulated vertebrae, and a few incomplete ribs. Collected by 

 M. V. Walker, 1931. 



Type locality. — A small badland area that is bisected by U. S. 

 Highway No. 20, about 8 miles east of Douglas, Converse County, 

 Wyo. 



Horizon. — Lower nodular layer of the Brule, Oligocene. 



Description. — The type specimen was found weathered out on the 

 surface of a nodule. The skull of this specimen appears to be the 

 smallest reptilian cranium that has yet been found in North America, 

 measuring only 7.8 mm in length. It is thus slightly shorter than 

 the type skull of Rhinsura stenibergii (see fig. 2) and also is less 

 robust in its other proportions. Both of these specimens were found 

 at the same locality, and the small size of the National Museum indi- 

 vidual led me at first to regard it as pertaining to R. stei-nhergii. 

 Through the courtesy of George F. Sternberg, who loaned me the 

 type, I was able to make direct comparisons of the two skulls, and 

 differences were found that indicate that they pertain to distinct 

 species. 



From R. sterribei^gii the skull of R. minutus is distinguished by the 

 less steeply arched profile, the absence of a distinct sagittal ridge, the 

 absence of roughening on the frontal and parietal surfaces, nar- 

 rower occipital region, slenderer maxillary, shorter precoronoidal 

 part of jaw, and longer postcoronoidal part. All these differences are 

 clearly seen by a comparison of figures 1 and 2. 



From R. hatcherii Baur, known from the Brule formation of South 

 Dakota and Nebraska, R. minutus is at once distinguished by its 

 much smaller size, being about one-half the dimensions of the known 

 skulls of that species. Furthermore, those characters enumerated 

 above tliat distinguished it from R. stemhergii also serve to differ- 

 entiate R. minutus from R. liatcherii. 



The absence of a sagittal ridge on the parietal and supraoccipital 

 and the lack of roughening of the frontal and parietal surfaces might 

 suggest the juvenile character of the individual, if it were not for the 

 fact that most of the skull sutures are so thoroughly coossified as to 

 defy detection. It is concluded, therefore, that the type specimen is 

 fully adult. 



In profile the upper border is evenly rounded from front to back, 

 differing in this respect from all known species of Rhineura both 

 living and extinct that have the facial and occipital angles much 

 more steeply inclined. This depression of the skull brings about a 



