194 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. XXXVI. 



plastron is 40 mm. long and 11 mm. wide. The hyoplastron and the 

 hypoplastron of the one side seem not to have come into contact with 

 those of the other. There is a large umbilical fontanel. 



The scapula?, S6, 57 are both present. The right coracoicl, 5 It, is 

 27 mm. long. The humeri, 5-5, -55, are each 27 mm. long. The head 

 of that of the right side appears behind a costal, 47; that of the left 

 humerus behind another costal, 39. The left radius and ulna are 

 seen between the scapula, J(>, and the humerus, 68. Some fore-foot 

 bones appear in front of the peripherals bearing the numbers 20 and 

 21. A phalanx, probably of the first digit, extends from the lower 

 jaw, t?, to the right parietal, -5. 



CHISTERNON? INTERPOSITUM, new species. 



The single known specimen of this species was collected during the 

 summer of 1908 by Mr. C. F. Kay, of the U. S. Geological Survey, 



in the Livingston coal field of 

 Montana. The formation is the 

 Fort ITnion. The more exact lo- 

 cality is given as T. 5 S., R. 19 E. 

 This is in Carbon County, about 

 10 or 15 miles west of north of 

 Red Lodge, and on or near some 

 of the sources of Red Lodge 

 Creek. The catalogue number 

 of this specimen in the U. S. 

 National Museum is 6058. 



The individual is represented 

 by parts of the anterior two- 

 thirds of both the carapace and 

 the plastron. Such parts as can 

 be fitted together are represented 

 by figs. 2 and 3. The other frag- 

 ments throw little light on the 

 characters of the species. The sutures between the various bones re- 

 mained open during life, and may now be followed without difficulty. 

 The species is referred with some doubt to the genus Chisternon, 

 hitherto known only from the Bridger. It possibly belongs to Bo- 

 remys Lambe, known hitherto onl}?^ from the Belly River beds of 

 Alberta, British America. As in both genera, there is present a pre- 

 neural bone. In Boremys there are supramarginal scutes, in Chis- 

 ternon none. It is possible that in this Fort Union species there 

 were such scutes, but there is little on which to found an opinion. 

 The distal extremity of one costal is present, and this shows no 

 indications of any sulcus crossing it. 



The individual was a rather large one, the width having been 

 about 300 mm. The length may have been somewhat greater. 



Fig. 2. — Chisternon ? intbkp o s i t d m . 

 Part op carapace. X |. c. p. 1, first 



COSTAL PLATE ; C. p. 3, THIRD COSTAL 

 PLATE ; n U, NBDRAL PLATE ; M. 1, FIRST 

 NEURAL PLATE ; n. l), FOURTH NEURAL 



plate; per.l, first peripheral ; pren, 



PRENEURAL BONK. 



