123 



quadrate or less triangular in form. The constitution is similar, but the outer 

 lobes are proportionately better developed and the median ones are much 

 reduced in size. A basal ridge nearly embraces the crown, but is nearly 

 obsolete internally, and is best developed posteriorly, where it forms a wide 

 festoon. 



The isolated tooth is a diminished representative of the one in the jaw- 

 fragment, and probably held the position of the third in advance of it. It 

 may, perhaps, represent a smaller species. The specimens indicate an animal 

 but little more than half the size of the opossum. How it is related with 

 Omomys the paucity of material prevents a positive determination. The size 

 of the teeth indicates a larger animal than Omomys Carteri. 



In the American Journal of Science for 1871, Professor Marsh has 

 described a tooth, from Grizzly Buttes, which he likens to the premolars of 

 some insectivora, and refers it to a species with the name of Trlacodon fallax. 



He remarks that the species was probably about two-thirds of the size of 

 the opossum, which dimensions would be too great for the animal we have 

 named Palceacodon verus. 



The sizes of the teeth referred to the latter are as follows: 



Lines. 



Space occupied by the penultimate and antepenultimate molars . . 4 



Breadth of penultimate molar 2 



Width of penultimate molar 2\ 



Breadth of last premolar 1£ 



Width of last premolar la 



WASHAKIUS. 



Washakius insignis. 



A jaw-fragment of a small animal recently sent to me by Dr. Carter is rep- 

 resented in Fig. 3, Plate XXVII, magnified three diameters. The specimen 

 was found in the Bridger beds by a Shoshone Indian and given to Dr. Carter. 

 It is quite different in appearance from any similar fossil from the same for- 

 mation submitted to my inspection, and appears to indicate a different genus 

 from those described in the preceding pages. I am uncertain as to its ordi- 

 nal affinities, but suspect it to have pertained to an insectivorous animal, per- 

 haps one of the many which have been indicated by Professor Marsh from fossils 

 of the Bridger beds. 



The jaw-fragment contains the last two molars, the triturating surfaces of 



