498 PROCEEDINGS OF THE NATIONAL UUSEUM. vol.38. 



Daw -on County, L35 miles northwest of Miles City, Montana, by 

 Mr. Barnum Brown, through whose courtesy I am now permitted to 

 describe it. 



The specimen is thai of an adult individual of slightly larger size 



than the type of the speeies. Like the latter, however, nearly all the 

 sutures of the facical portion of the skull are obliterated and we must 

 await the discovery of other material before the relative relationships 

 of these elements can be determined. 



Compared with the type of the speeies, it differs in the greater 

 breadth of the muzzle, the larger size of the teeth, the flatness of the 

 facical region without the upturn of the premaxillary part, and the 

 uniform coarseness of the sculpt uring of the superior aspect. At first 

 I was inclined to believe the differences enumerated were sufficient to 



justify the establishment of a new species, but after a careful study 

 of a series of recent crocodile and alligator crania, and noting the 

 occurrence of essentially the same differences in skulls of indi- 

 viduals collected from the same region, and undoubtedlv belonging 

 to the same species, there appeared no warrant for so doing. Such 

 characters as have been mentioned can all be accounted for, in recent 

 forms, by individual variation due in most part to differences in age, 

 and it would appear reasonable to suppose these observations would 

 also apply to the fossil members of this group. 



The dental formula of the cranium is the same as in the type — that 

 is, five premaxillary and nineteen maxillary teeth. The ends of the 

 premaxillary processes on the palate are more broadly rounded than 

 in the type. 



The difference in the contour of the muzzles of the two skulls is 

 apparently due to the difference in age of the two specimens, the 

 latter, as shown by its larger size and complete obliteration of most 

 of the sutures, being considered the more mature. 



In this individual the palatines are complete, though their line oi 

 union with the pterygoids can not be made out. Those parts of 

 the pterygoids -till attached to the posterior ends of the palatines 

 (shown in PI, 29) are most important as giving the shape and posi- 

 tion of the posterior nares. This aperture in L< idyosuchus appears to 

 have been wholly surrounded by the horizontal plate of the ptery- 

 goids. In outline (see p. n. fig. 29) it may besl be described as heart- 

 shaped with the apex directed backward. While it resembles the 

 posterior nares in Diplocynodon as figured by Owen." its position, as 

 would be expected from their relative geological positidhs, is con- 

 siderably more forward on the palate. Measured from a line drawn 

 transversely across the back borders of the posterior palatine vacuities, 

 t he anterior border of the nares is 1 I nun. posterior to it. 



Monograph of the fossil Reptilia of the London Clay, pt. 2, L850, pi. 7. fig. 2. 



